nfs: fix oops when trying to set SELinux label

Chao reported the following oops when testing labeled NFS:

BUG: unable to handle kernel NULL pointer dereference at           (null)
IP: [<ffffffffa0568703>] nfs4_xdr_enc_setattr+0x43/0x110 [nfsv4]
PGD 277bbd067 PUD 2777ea067 PMD 0
Oops: 0000 [#1] SMP
Modules linked in: rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache sg coretemp kvm_intel kvm crc32_pclmul crc32c_intel ghash_clmulni_intel aesni_intel lrw gf128mul iTCO_wdt glue_helper ablk_helper cryptd iTCO_vendor_support bnx2 pcspkr serio_raw i7core_edac cdc_ether microcode usbnet edac_core mii lpc_ich i2c_i801 mfd_core shpchp ioatdma dca acpi_cpufreq mperf nfsd auth_rpcgss nfs_acl lockd sunrpc xfs libcrc32c sr_mod sd_mod cdrom crc_t10dif mgag200 syscopyarea sysfillrect sysimgblt i2c_algo_bit drm_kms_helper ata_generic ttm pata_acpi drm ata_piix libata megaraid_sas i2c_core dm_mirror dm_region_hash dm_log dm_mod
CPU: 4 PID: 25657 Comm: chcon Not tainted 3.10.0-33.el7.x86_64 #1
Hardware name: IBM System x3550 M3 -[7944OEJ]-/90Y4784     , BIOS -[D6E150CUS-1.11]- 02/08/2011
task: ffff880178397220 ti: ffff8801595d2000 task.ti: ffff8801595d2000
RIP: 0010:[<ffffffffa0568703>]  [<ffffffffa0568703>] nfs4_xdr_enc_setattr+0x43/0x110 [nfsv4]
RSP: 0018:ffff8801595d3888  EFLAGS: 00010296
RAX: 0000000000000000 RBX: ffff8801595d3b30 RCX: 0000000000000b4c
RDX: ffff8801595d3b30 RSI: ffff8801595d38e0 RDI: ffff880278b6ec00
RBP: ffff8801595d38c8 R08: ffff8801595d3b30 R09: 0000000000000001
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801595d38e0
R13: ffff880277a4a780 R14: ffffffffa05686c0 R15: ffff8802765f206c
FS:  00007f2c68486800(0000) GS:ffff88027fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000027651a000 CR4: 00000000000007e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Stack:
 0000000000000000 0000000000000000 0000000000000000 0000000000000000
 0000000000000000 ffff880277865800 ffff880278b6ec00 ffff880277a4a780
 ffff8801595d3948 ffffffffa02ad926 ffff8801595d3b30 ffff8802765f206c
Call Trace:
 [<ffffffffa02ad926>] rpcauth_wrap_req+0x86/0xd0 [sunrpc]
 [<ffffffffa02a1d40>] ? call_connect+0xb0/0xb0 [sunrpc]
 [<ffffffffa02a1d40>] ? call_connect+0xb0/0xb0 [sunrpc]
 [<ffffffffa02a1ecb>] call_transmit+0x18b/0x290 [sunrpc]
 [<ffffffffa02a1d40>] ? call_connect+0xb0/0xb0 [sunrpc]
 [<ffffffffa02aae14>] __rpc_execute+0x84/0x400 [sunrpc]
 [<ffffffffa02ac40e>] rpc_execute+0x5e/0xa0 [sunrpc]
 [<ffffffffa02a2ea0>] rpc_run_task+0x70/0x90 [sunrpc]
 [<ffffffffa02a2f03>] rpc_call_sync+0x43/0xa0 [sunrpc]
 [<ffffffffa055284d>] _nfs4_do_set_security_label+0x11d/0x170 [nfsv4]
 [<ffffffffa0558861>] nfs4_set_security_label.isra.69+0xf1/0x1d0 [nfsv4]
 [<ffffffff815fca8b>] ? avc_alloc_node+0x24/0x125
 [<ffffffff815fcd2f>] ? avc_compute_av+0x1a3/0x1b5
 [<ffffffffa055897b>] nfs4_xattr_set_nfs4_label+0x3b/0x50 [nfsv4]
 [<ffffffff811bc772>] generic_setxattr+0x62/0x80
 [<ffffffff811bcfc3>] __vfs_setxattr_noperm+0x63/0x1b0
 [<ffffffff811bd1c5>] vfs_setxattr+0xb5/0xc0
 [<ffffffff811bd2fe>] setxattr+0x12e/0x1c0
 [<ffffffff811a4d22>] ? final_putname+0x22/0x50
 [<ffffffff811a4f2b>] ? putname+0x2b/0x40
 [<ffffffff811aa1cf>] ? user_path_at_empty+0x5f/0x90
 [<ffffffff8119bc29>] ? __sb_start_write+0x49/0x100
 [<ffffffff811bd66f>] SyS_lsetxattr+0x8f/0xd0
 [<ffffffff8160cf99>] system_call_fastpath+0x16/0x1b
Code: 48 8b 02 48 c7 45 c0 00 00 00 00 48 c7 45 c8 00 00 00 00 48 c7 45 d0 00 00 00 00 48 c7 45 d8 00 00 00 00 48 c7 45 e0 00 00 00 00 <48> 8b 00 48 8b 00 48 85 c0 0f 84 ae 00 00 00 48 8b 80 b8 03 00
RIP  [<ffffffffa0568703>] nfs4_xdr_enc_setattr+0x43/0x110 [nfsv4]
 RSP <ffff8801595d3888>
CR2: 0000000000000000

The problem is that _nfs4_do_set_security_label calls rpc_call_sync()
directly which fails to do any setup of the SEQUENCE call. Have it use
nfs4_call_sync() instead which does the right thing. While we're at it
change the name of "args" to "arg" to better match the pattern in
_nfs4_do_setattr.

Reported-by: Chao Ye <cye@redhat.com>
Cc: David Quigley <dpquigl@davequigley.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: stable@vger.kernel.org # 3.11+
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>