nfs: set hostname when creating nfsv3 ds connection

lockd assumes hostname exists otherwise kernel oops.
It can be reproduced by following steps:
1. mount flexfile MDS
2. write some files
3. mount DS via nfsv3

BUG: unable to handle kernel NULL pointer dereference at           (null)
 IP: [<ffffffff8134f332>] strlen+0x2/0x20
 PGD 0
 Oops: 0000 [#1] SMP
 Modules linked in: nfsd(F) nfs_layout_flexfiles(F) rpcsec_gss_krb5(F) auth_rpcgss(F) nfsv4(F) dns_resolver(F) nfsv3(F) nfs_acl(F) nfs(F) lockd(F) sunrpc(F) fscache(F) ebtable_nat(F) nf_conntrack_netbios_ns(F) nf_conntrack_broadcast(F) ipt_MASQUERADE(F) ip6table_nat(F) nf_nat_ipv6(F) ip6table_mangle(F) ip6t_REJECT(F) nf_conntrack_ipv6(F) nf_defrag_ipv6(F) iptable_nat(F) nf_nat_ipv4(F) nf_nat(F) iptable_mangle(F) nf_conntrack_ipv4(F) nf_defrag_ipv4(F) xt_conntrack(F) nf_conntrack(F) ebtable_filter(F) ebtables(F) ip6table_filter(F) ip6_tables(F) bnep(F) snd_ens1371(F) snd_rawmidi(F) snd_ac97_codec(F) btusb(F) ac97_bus(F) snd_seq(F) snd_seq_device(F) snd_pcm(F) ppdev(F) bluetooth(F) 6lowpan_iphc(F) rfkill(F) vmw_balloon(F) snd_timer(F) snd(F) soundcore(F) gameport(F) i2c_piix4(F) e1000(F) vmw_vmci(F) parport_pc(F) parport(F) shpchp(F) uinput(F) xfs(F) libcrc32c(F) vmwgfx(F) ttm(F) drm(F) mptspi(F) scsi_transport_spi(F) mptscsih(F) mptbase(F) i2c_core(F)
 CPU: 0 PID: 10397 Comm: mount.nfs Tainted: GF            3.14.7-100.pd_client.001.fc16.x86_64 #1
 Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/31/2013
 task: ffff880008942600 ti: ffff880007990000 task.ti: ffff880007990000
 RIP: 0010:[<ffffffff8134f332>]  [<ffffffff8134f332>] strlen+0x2/0x20
 RSP: 0018:ffff880007991aa0  EFLAGS: 00010246
 RAX: 0000000000000000 RBX: ffff880038d39c20 RCX: 0000000000000004
 RDX: 0000000000000006 RSI: 0000000000000010 RDI: 0000000000000000
 RBP: ffff880007991b38 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000014600 R11: 0000000000000400 R12: ffffffff81cc8580
 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000004
 FS:  00007f90cd2ef880(0000) GS:ffff88003f600000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000000 CR3: 0000000001710000 CR4: 00000000001407f0
 Stack:
  ffffffffa045f52c ffff880001782230 ffff880004141e28 0006880007991ac8
  ffffffff816dc14b ffff880000000000 ffff880038d39c20 0000000000000010
  0000000481cc0006 0000000000000000 ffffffffa0410be8 000000000000c014
 Call Trace:
  [<ffffffffa045f52c>] ? nlmclnt_lookup_host+0x4c/0x2c0 [lockd]
  [<ffffffff816dc14b>] ? _raw_spin_unlock_bh+0x1b/0x20
  [<ffffffffa0410be8>] ? svc_destroy+0xb8/0x140 [sunrpc]
  [<ffffffffa045c323>] nlmclnt_init+0x53/0xc0 [lockd]
  [<ffffffffa047d2dc>] ? nfs_get_client+0x1cc/0x340 [nfs]
  [<ffffffffa047c2e7>] nfs_start_lockd+0xa7/0xd0 [nfs]
  [<ffffffffa047df71>] nfs_create_server+0x181/0x5c0 [nfs]
  [<ffffffffa04460f3>] nfs3_create_server+0x13/0x30 [nfsv3]
  [<ffffffffa048a0bc>] nfs_try_mount+0x21c/0x300 [nfs]
  [<ffffffff811ca32d>] ? __kmalloc_track_caller+0x1ad/0x240
  [<ffffffffa048b677>] ? nfs_fs_mount+0xc37/0xd80 [nfs]
  [<ffffffffa048ad05>] nfs_fs_mount+0x2c5/0xd80 [nfs]
  [<ffffffffa048a830>] ? nfs_clone_super+0x140/0x140 [nfs]
  [<ffffffffa048a240>] ? nfs_clone_sb_security+0x40/0x40 [nfs]
  [<ffffffff811e7e43>] mount_fs+0x43/0x1b0
  [<ffffffff81193100>] ? __alloc_percpu+0x10/0x20
  [<ffffffff812026e6>] vfs_kern_mount+0x76/0x120
  [<ffffffff81204917>] do_mount+0x237/0xa80

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>