Before the nl REMOVE msg has been sent to the userspace, the ring's
and other resources have been released, but the userspace maybe still
using them. And then we can see the crash messages like:

ring broken, not handling completions
BUG: unable to handle kernel paging request at ffffffffffffffd0
IP: tcmu_handle_completions+0x134/0x2f0 [target_core_user]
PGD 11bdc0c067
P4D 11bdc0c067
PUD 11bdc0e067
PMD 0

Oops: 0000 [#1] SMP
cmd_id not found, ring is broken
RIP: 0010:tcmu_handle_completions+0x134/0x2f0 [target_core_user]
RSP: 0018:ffffb8a2d8983d88 EFLAGS: 00010296
RAX: 0000000000000000 RBX: ffffb8a2aaa4e000 RCX: 00000000ffffffff
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000220
R10: 0000000076c71401 R11: ffff8d2e76c713f0 R12: ffffb8a2aad56bc0
R13: 000000000000001c R14: ffff8d2e32c90000 R15: ffff8d2e76c713f0
FS:  00007f411ffff700(0000) GS:ffff8d1e7fdc0000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd0 CR3: 0000001027070000 CR4:
00000000001406e0
Call Trace:
? tcmu_irqcontrol+0x2a/0x40 [target_core_user]
? uio_write+0x7b/0xc0 [uio]
? __vfs_write+0x37/0x150
? __getnstimeofday64+0x3b/0xd0
? vfs_write+0xb2/0x1b0
? syscall_trace_enter+0x1d0/0x2b0
? SyS_write+0x55/0xc0
? do_syscall_64+0x67/0x150
? entry_SYSCALL64_slow_path+0x25/0x25
Code: 41 5d 41 5e 41 5f 5d c3 83 f8 01 0f 85 cf 01 00
00 48 8b 7d d0 e8 dd 5c 1d f3 41 0f b7 74 24 04 48 8b
7d c8 31 d2 e8 5c c7 1b f3 <48> 8b 7d d0 49 89 c7 c6 07
00 0f 1f 40 00 4d 85 ff 0f 84 82 01  RIP:
tcmu_handle_completions+0x134/0x2f0 [target_core_user]
RSP: ffffb8a2d8983d88
CR2: ffffffffffffffd0

And the crash also could happen in tcmu_page_fault and other places.
Signed-off-by: Zhang Zhuoyu <zhangzhuoyu@cmss.chinamobile.com>
Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>