When lseg's commit_through_mds is set, pnfs client always WARN once
in nfs_direct_select_verf after checking ds_cinfo.nbuckets.

nfs should use the DS verf except commit_through_mds is set for
layout segment where nbuckets is zero.

[17844.666094] ------------[ cut here ]------------
[17844.667071] WARNING: CPU: 0 PID: 21758 at /root/source/linux-pnfs/fs/nfs/direct.c:174 nfs_direct_select_verf+0x5a/0x70 [nfs]()
[17844.668650] Modules linked in: nfs_layout_nfsv41_files(OE) nfsv4(OE) nfs(OE) fscache(E) nfsd(OE) xfs libcrc32c btrfs ppdev coretemp crct10dif_pclmul auth_rpcgss crc32_pclmul crc32c_intel nfs_acl ghash_clmulni_intel lockd vmw_balloon xor vmw_vmci grace raid6_pq shpchp sunrpc parport_pc i2c_piix4 parport vmwgfx drm_kms_helper ttm drm serio_raw mptspi e1000 scsi_transport_spi mptscsih mptbase ata_generic pata_acpi [last unloaded: fscache]
[17844.686676] CPU: 0 PID: 21758 Comm: kworker/0:1 Tainted: G        W  OE   4.3.0-rc1-pnfs+ #245
[17844.687352] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 05/20/2014
[17844.698502] Workqueue: nfsiod rpc_async_release [sunrpc]
[17844.699212]  0000000000000009 0000000043e58010 ffff8800454fbc10 ffffffff813680c4
[17844.699990]  ffff8800454fbc48 ffffffff8108b49d ffff88004eb20000 ffff88004eb20000
[17844.700844]  ffff880062e26000 0000000000000000 0000000000000001 ffff8800454fbc58
[17844.701637] Call Trace:
[17844.725252]  [<ffffffff813680c4>] dump_stack+0x19/0x25
[17844.732693]  [<ffffffff8108b49d>] warn_slowpath_common+0x7d/0xb0
[17844.733855]  [<ffffffff8108b5da>] warn_slowpath_null+0x1a/0x20
[17844.735015]  [<ffffffffa04a27ca>] nfs_direct_select_verf+0x5a/0x70 [nfs]
[17844.735999]  [<ffffffffa04a2b83>] nfs_direct_set_hdr_verf+0x23/0x90 [nfs]
[17844.736846]  [<ffffffffa04a2e17>] nfs_direct_write_completion+0x227/0x260 [nfs]
[17844.737782]  [<ffffffffa04a433c>] nfs_pgio_release+0x1c/0x20 [nfs]
[17844.738597]  [<ffffffffa0502df3>] pnfs_generic_rw_release+0x23/0x30 [nfsv4]
[17844.739486]  [<ffffffffa01cbbea>] rpc_free_task+0x2a/0x70 [sunrpc]
[17844.740326]  [<ffffffffa01cbcd5>] rpc_async_release+0x15/0x20 [sunrpc]
[17844.741173]  [<ffffffff810a387c>] process_one_work+0x21c/0x4c0
[17844.741984]  [<ffffffff810a37cd>] ? process_one_work+0x16d/0x4c0
[17844.742837]  [<ffffffff810a3b6a>] worker_thread+0x4a/0x440
[17844.743639]  [<ffffffff810a3b20>] ? process_one_work+0x4c0/0x4c0
[17844.744399]  [<ffffffff810a3b20>] ? process_one_work+0x4c0/0x4c0
[17844.745176]  [<ffffffff810a8d75>] kthread+0xf5/0x110
[17844.745927]  [<ffffffff810a8c80>] ? kthread_create_on_node+0x240/0x240
[17844.747105]  [<ffffffff8172ce1f>] ret_from_fork+0x3f/0x70
[17844.747856]  [<ffffffff810a8c80>] ? kthread_create_on_node+0x240/0x240
[17844.748642] ---[ end trace 336a2845d42b83f0 ]---

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>