LU-6794 nfs: ASSERTION( atomic_read(&d->ld_ref) == 0 ) failed

LU-6794 nfs: ASSERTION( atomic_read(&d->ld_ref) == 0 ) failed

ll_iget_for_nfs() can call unbalanced iput() causing memory
leaks. This patch removes this iput() call.

Also, avoid unhashing disconnected dentries in
d_lustre_invalidate(), which is another source of memory
leaks.

One of the symptoms of the leak is the following crash pattern:
<0>LustreError: 14812:0:(lu_object.c:1251:lu_device_fini())
ASSERTION( atomic_read(&d->ld_ref) == 0 ) failed: Refcount is 1
<0>LustreError: 14812:0:(lu_object.c:1251:lu_device_fini()) LBUG
<4>Pid: 14812, comm: umount
<4>
<4>Call Trace:
<4> [<ffffffffa11bc895>] libcfs_debug_dumpstack+0x55/0x80 [libcfs]
<4> [<ffffffffa11bce97>] lbug_with_loc+0x47/0xb0 [libcfs]
<4> [<ffffffffa1458a48>] lu_device_fini+0xb8/0xc0 [obdclass]
<4> [<ffffffffa08e9ab2>] lovsub_device_free+0x52/0x220 [lov]
<4> [<ffffffffa145c64e>] lu_stack_fini+0x7e/0xc0 [obdclass]
<4> [<ffffffffa146356e>] cl_stack_fini+0xe/0x10 [obdclass]
<4> [<ffffffffa08bc1a8>] lov_device_fini+0x58/0x120 [lov]
<4> [<ffffffffa145c619>] lu_stack_fini+0x49/0xc0 [obdclass]
<4> [<ffffffffa146356e>] cl_stack_fini+0xe/0x10 [obdclass]
<4> [<ffffffffa0e1279d>] cl_sb_fini+0x6d/0x190 [lustre]
<4> [<ffffffffa0dd34bc>] ll_put_super+0x1bc/0x11e0 [lustre]
<4> [<ffffffff811cd0f2>] ? fsnotify_clear_marks_by_inode+0x32/0xf0
<4> [<ffffffff811a59df>] ? destroy_inode+0x2f/0x60
<4> [<ffffffff811a5eac>] ? dispose_list+0xfc/0x120
<4> [<ffffffff811a62a6>] ? invalidate_inodes+0xf6/0x190
<4> [<ffffffff8118b35b>] generic_shutdown_super+0x5b/0xe0
<4> [<ffffffff8118b446>] kill_anon_super+0x16/0x60
<4> [<ffffffffa144e7ba>] lustre_kill_super+0x4a/0x60 [obdclass]
<4> [<ffffffff8118bbe7>] deactivate_super+0x57/0x80
<4> [<ffffffff811aabef>] mntput_no_expire+0xbf/0x110
<4> [<ffffffff811ab73b>] sys_umount+0x7b/0x3a0
<4> [<ffffffff8100b072>] system_call_fastpath+0x16/0x1b

Xyratex-bug-id: MRP-2414
Signed-off-by: Andrew Perepechko <andrew.perepechko@seagate.com>
Change-Id: Ie8bc428014f3897dc108568f55db9a9ed93c932b
Reviewed-on: http://review.whamcloud.com/15480
Tested-by: Jenkins
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>