NFSD: Put the reference of nfs4_file when freeing stid

After testing nfs4 lock, I restart the nfsd service, got messages as,

[ 5677.403419] nfsd: last server has exited, flushing export cache
[ 5677.463728] =============================================================================
[ 5677.463942] BUG nfsd4_files (Tainted: G    B      OE): Objects remaining in nfsd4_files on kmem_cache_close()
[ 5677.464055] -----------------------------------------------------------------------------

[ 5677.464203] INFO: Slab 0xffffea0000233400 objects=28 used=1 fp=0xffff880008cd3d98 flags=0x3ffc0000004080
[ 5677.464318] CPU: 0 PID: 3772 Comm: rmmod Tainted: G    B      OE 3.16.0-rc2+ #29
[ 5677.464420] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/31/2013
[ 5677.464538]  0000000000000000 0000000036af2c9f ffff88000ce97d68 ffffffff816eacfa
[ 5677.464643]  ffffea0000233400 ffff88000ce97e40 ffffffff811cda44 ffffffff00000020
[ 5677.464774]  ffff88000ce97e50 ffff88000ce97e00 656a624f00000008 616d657220737463
[ 5677.464875] Call Trace:
[ 5677.464925]  [<ffffffff816eacfa>] dump_stack+0x45/0x56
[ 5677.464983]  [<ffffffff811cda44>] slab_err+0xb4/0xe0
[ 5677.465040]  [<ffffffff811d0457>] ? __kmalloc+0x117/0x290
[ 5677.465099]  [<ffffffff81100eec>] ? on_each_cpu_cond+0xac/0xf0
[ 5677.465158]  [<ffffffff811d1bc0>] ? kmem_cache_close+0x110/0x2e0
[ 5677.465218]  [<ffffffff811d1be0>] kmem_cache_close+0x130/0x2e0
[ 5677.465279]  [<ffffffff8135a0c1>] ? kobject_cleanup+0x91/0x1b0
[ 5677.465338]  [<ffffffff811d22be>] __kmem_cache_shutdown+0xe/0x10
[ 5677.465399]  [<ffffffff8119bd28>] kmem_cache_destroy+0x48/0x100
[ 5677.465466]  [<ffffffffa05ef78d>] nfsd4_free_slabs+0x2d/0x50 [nfsd]
[ 5677.465530]  [<ffffffffa05fa987>] exit_nfsd+0x34/0x6ad [nfsd]
[ 5677.465589]  [<ffffffff81104ac2>] SyS_delete_module+0x162/0x200
[ 5677.465649]  [<ffffffff81013b69>] ? do_notify_resume+0x59/0x90
[ 5677.465759]  [<ffffffff816f2369>] system_call_fastpath+0x16/0x1b
[ 5677.465822] INFO: Object 0xffff880008cd0000 @offset=0
[ 5677.465882] INFO: Allocated in nfsd4_process_open1+0x61/0x350 [nfsd] age=7599 cpu=0 pid=3253
[ 5677.466115]  __slab_alloc+0x3b0/0x4b1
[ 5677.466166]  kmem_cache_alloc+0x1e4/0x240
[ 5677.466220]  nfsd4_process_open1+0x61/0x350 [nfsd]
[ 5677.466276]  nfsd4_open+0xee/0x860 [nfsd]
[ 5677.466329]  nfsd4_proc_compound+0x4d7/0x7f0 [nfsd]
[ 5677.466384]  nfsd_dispatch+0xbb/0x200 [nfsd]
[ 5677.466447]  svc_process_common+0x453/0x6f0 [sunrpc]
[ 5677.466506]  svc_process+0x103/0x170 [sunrpc]
[ 5677.466559]  nfsd+0x117/0x190 [nfsd]
[ 5677.466609]  kthread+0xd8/0xf0
[ 5677.466656]  ret_from_fork+0x7c/0xb0
[ 5677.466775] kmem_cache_destroy nfsd4_files: Slab cache still has objects
[ 5677.466839] CPU: 0 PID: 3772 Comm: rmmod Tainted: G    B      OE 3.16.0-rc2+ #29
[ 5677.466937] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/31/2013
[ 5677.467049]  0000000000000000 0000000036af2c9f ffff88000ce97eb0 ffffffff816eacfa
[ 5677.467150]  ffff880020bb2d00 ffff88000ce97ed0 ffffffff8119bdd9 0000000000000000
[ 5677.467250]  ffffffffa06065c0 ffff88000ce97ee0 ffffffffa05ef78d ffff88000ce97ef0
[ 5677.467351] Call Trace:
[ 5677.467397]  [<ffffffff816eacfa>] dump_stack+0x45/0x56
[ 5677.467454]  [<ffffffff8119bdd9>] kmem_cache_destroy+0xf9/0x100
[ 5677.467516]  [<ffffffffa05ef78d>] nfsd4_free_slabs+0x2d/0x50 [nfsd]
[ 5677.467579]  [<ffffffffa05fa987>] exit_nfsd+0x34/0x6ad [nfsd]
[ 5677.467639]  [<ffffffff81104ac2>] SyS_delete_module+0x162/0x200
[ 5677.467765]  [<ffffffff81013b69>] ? do_notify_resume+0x59/0x90
[ 5677.467826]  [<ffffffff816f2369>] system_call_fastpath+0x16/0x1b

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Reviewed-by: Jeff Layton <jlayton@primarydata.com>
Fixes: 11b9164a "nfsd: Add a struct nfs4_file field to struct nfs4_stid"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>