VxFS umount caused kernel panic in deactive_super() function provided by third party module vxfs.

Issue

• VxFS umount panicked in deactive_super() function, with the following call trace :

Call Trace:
 [<ffffffffa04f0304>] vx_fsnotify_flush+0x24/0x50 [vxfs]
 [<ffffffffa047f3a8>] vx_softcnt_flush+0xb8/0x480 [vxfs]
 [<ffffffffa042d914>] vx_idrop+0x104/0x560 [vxfs]
 [<ffffffffa0467744>] vx_detach_fset+0x1a34/0x1ee0 [vxfs]
 [<ffffffffa0f08c00>] ? vxg_lock_fastsub+0x0/0xe0 [vxglm]
 [<ffffffffa03ef065>] ? vx_glmref_lock+0x35/0x40 [vxfs]
 [<ffffffffa04f0b4d>] vx_do_unmount+0x1ed/0x5c0 [vxfs]
 [<ffffffffa04f0f55>] vx_unmount+0x35/0x90 [vxfs]
 [<ffffffff8118380b>] generic_shutdown_super+0x5b/0xe0
 [<ffffffff811838c1>] kill_block_super+0x31/0x50
 [<ffffffffa04ef667>] vx_kill_sb+0x47/0x170 [vxfs]
 [<ffffffffa0f741ca>] amf_kill_sb+0xaa/0x150 [amf]
 [<ffffffff81184097>] deactivate_super+0x57/0x80
 [<ffffffff811a234f>] mntput_no_expire+0xbf/0x110
 [<ffffffff811a2dbb>] sys_umount+0x7b/0x3a0
 [<ffffffff8100b072>] system_call_fastpath+0x16/0x1b
Code: c8 e9 45 ff ff ff 66 66 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 48 83 ec 10 48 89 1c 24 4c 89 64 24 08 0f 1f 44 00 00 <4c> 8b 67 30 48 89 fb f0 ff 8f b8 00 00 00 0f 94 c0 84 c0 74 3c 
RIP  [<ffffffff81184056>] deactivate_super+0x16/0x80