pnfs: fix lockup caused by pnfs_generic_pg_test

pnfs: fix lockup caused by pnfs_generic_pg_test

end_offset and req_offset both return u64 - avoid casting to u32
until it's needed, when it's less than the (u32) size returned by
nfs_generic_pg_test.

Also, fix the comments in pnfs_generic_pg_test.

Running the cthon04 special tests caused this lockup in the
"write/read at 2GB, 4GB edges" test when running against a file layout server:

BUG: soft lockup - CPU#0 stuck for 22s! [bigfile2:823]
Modules linked in: nfs_layout_nfsv41_files rpcsec_gss_krb5 nfsv4 nfs fscache ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_mangle ip6table_filter ip6_tables iptable_nat nf_nat_ipv4 nf_nat iptable_mangle ppdev crc32c_intel aesni_intel aes_x86_64 glue_helper lrw gf128mul ablk_helper cryptd serio_raw e1000 shpchp i2c_piix4 i2c_core parport_pc parport nfsd auth_rpcgss oid_registry exportfs nfs_acl lockd sunrpc btrfs xor zlib_deflate raid6_pq mptspi scsi_transport_spi mptscsih mptbase ata_generic floppy autofs4
irq event stamp: 205958
hardirqs last  enabled at (205957): [<ffffffff814a62dc>] restore_args+0x0/0x30
hardirqs last disabled at (205958): [<ffffffff814ad96a>] apic_timer_interrupt+0x6a/0x80
softirqs last  enabled at (205956): [<ffffffff8103ffb2>] __do_softirq+0x1ea/0x2ab
softirqs last disabled at (205951): [<ffffffff8104026d>] irq_exit+0x44/0x9a
CPU: 0 PID: 823 Comm: bigfile2 Not tainted 3.15.0-rc1-branch-pgio_plus+ #3
Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/31/2013
task: ffff8800792ec480 ti: ffff880078c4e000 task.ti: ffff880078c4e000
RIP: 0010:[<ffffffffa02ce51f>]  [<ffffffffa02ce51f>] nfs_page_group_unlock+0x3e/0x4b [nfs]
RSP: 0018:ffff880078c4fab0  EFLAGS: 00000202
RAX: 0000000000000fff RBX: ffff88006bf83300 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffff88006bf83300
RBP: ffff880078c4fab8 R08: 0000000000000001 R09: 0000000000000000
R10: ffffffff8249840c R11: 0000000000000000 R12: 0000000000000035
R13: ffff88007ffc72d8 R14: 0000000000000001 R15: 0000000000000000
FS:  00007f45f11b7740(0000) GS:ffff88007f200000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f3a8cb632d0 CR3: 000000007931c000 CR4: 00000000001407f0
Stack:
 ffff88006bf832c0 ffff880078c4fb00 ffffffffa02cec22 ffff880078c4fad8
 00000fff810f9d99 ffff880078c4fca0 ffff88006bf832c0 ffff88006bf832c0
 ffff880078c4fca0 ffff880078c4fd60 ffff880078c4fb28 ffffffffa02cee34
Call Trace:
 [<ffffffffa02cec22>] __nfs_pageio_add_request+0x298/0x34f [nfs]
 [<ffffffffa02cee34>] nfs_pageio_add_request+0x1f/0x42 [nfs]
 [<ffffffffa02d1722>] nfs_do_writepage+0x1b5/0x1e4 [nfs]
 [<ffffffffa02d1764>] nfs_writepages_callback+0x13/0x25 [nfs]
 [<ffffffffa02d1751>] ? nfs_do_writepage+0x1e4/0x1e4 [nfs]
 [<ffffffff810eb32d>] write_cache_pages+0x254/0x37f
 [<ffffffffa02d1751>] ? nfs_do_writepage+0x1e4/0x1e4 [nfs]
 [<ffffffff8149cf9e>] ? printk+0x54/0x56
 [<ffffffff810eacca>] ? __set_page_dirty_nobuffers+0x22/0xe9
 [<ffffffffa016d864>] ? put_rpccred+0x38/0x101 [sunrpc]
 [<ffffffffa02d1ae1>] nfs_writepages+0xb4/0xf8 [nfs]
 [<ffffffff810ec59c>] do_writepages+0x21/0x2f
 [<ffffffff810e36e8>] __filemap_fdatawrite_range+0x55/0x57
 [<ffffffff810e374a>] filemap_write_and_wait_range+0x2d/0x5b
 [<ffffffffa030ba0a>] nfs4_file_fsync+0x3a/0x98 [nfsv4]
 [<ffffffff8114ee3c>] vfs_fsync_range+0x18/0x20
 [<ffffffff810e40c2>] generic_file_aio_write+0xa7/0xbd
 [<ffffffffa02c5c6b>] nfs_file_write+0xf0/0x170 [nfs]
 [<ffffffff81129215>] do_sync_write+0x59/0x78
 [<ffffffff8112956c>] vfs_write+0xab/0x107
 [<ffffffff81129c8b>] SyS_write+0x49/0x7f
 [<ffffffff814acd12>] system_call_fastpath+0x16/0x1b

Reported-by: Anna Schumaker <Anna.Schumaker@netapp.com>
Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>