net/mlx5e: nullify cq->dbg pointer in mlx5_debug_cq_remove()

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5e: nullify cq->dbg pointer in mlx5_debug_cq_remove()

Prior to this patch in case mlx5_core_destroy_cq() failed it proceeds
to rest of destroy operations. mlx5_core_destroy_cq() could be called again
by user and cause additional call of mlx5_debug_cq_remove().
cq->dbg was not nullify in previous call and cause the crash.

Fix it by nullify cq->dbg pointer after removal.

Also proceed to destroy operations only if FW return 0
for MLX5_CMD_OP_DESTROY_CQ command.

general protection fault, probably for non-canonical address 0x2000300004058: 0000 [#1] SMP PTI
CPU: 5 PID: 1228 Comm: python Not tainted 5.15.0-rc5_for_upstream_min_debug_2021_10_14_11_06 #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
RIP: 0010:lockref_get+0x1/0x60
Code: 5d e9 53 ff ff ff 48 8d 7f 70 e8 0a 2e 48 00 c7 85 d0 00 00 00 02
00 00 00 c6 45 70 00 fb 5d c3 c3 cc cc cc cc cc cc cc cc 53 <48> 8b 17
48 89 fb 85 d2 75 3d 48 89 d0 bf 64 00 00 00 48 89 c1 48
RSP: 0018:ffff888137dd7a38 EFLAGS: 00010206
RAX: 0000000000000000 RBX: ffff888107d5f458 RCX: 00000000fffffffe
RDX: 000000000002c2b0 RSI: ffffffff8155e2e0 RDI: 0002000300004058
RBP: ffff888137dd7a88 R08: 0002000300004058 R09: ffff8881144a9f88
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8881141d4000
R13: ffff888137dd7c68 R14: ffff888137dd7d58 R15: ffff888137dd7cc0
FS: 00007f4644f2a4c0(0000) GS:ffff8887a2d40000(0000)
knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055b4500f4380 CR3: 0000000114f7a003 CR4: 0000000000170ea0
Call Trace:
simple_recursive_removal+0x33/0x2e0
? debugfs_remove+0x60/0x60
debugfs_remove+0x40/0x60
mlx5_debug_cq_remove+0x32/0x70 [mlx5_core]
mlx5_core_destroy_cq+0x41/0x1d0 [mlx5_core]
devx_obj_cleanup+0x151/0x330 [mlx5_ib]
? __pollwait+0xd0/0xd0
? xas_load+0x5/0x70
? xa_load+0x62/0xa0
destroy_hw_idr_uobject+0x20/0x80 [ib_uverbs]
uverbs_destroy_uobject+0x3b/0x360 [ib_uverbs]
uobj_destroy+0x54/0xa0 [ib_uverbs]
ib_uverbs_cmd_verbs+0xaf2/0x1160 [ib_uverbs]
? uverbs_finalize_object+0xd0/0xd0 [ib_uverbs]
ib_uverbs_ioctl+0xc4/0x1b0 [ib_uverbs]
__x64_sys_ioctl+0x3e4/0x8e0