Avoid that scsi_show_rq() triggers a NULL pointer dereference if called
after sd_uninit_command(). Swap the NULL pointer assignment and the
mempool_free() call in sd_uninit_command() to make it less likely that
scsi_show_rq() triggers a use-after-free. Note: even with these changes
scsi_show_rq() can trigger a use-after-free but that's a lesser evil
than e.g. suppressing debug information for T10 PI Type 2 commands
completely. This patch fixes the following oops:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: scsi_format_opcode_name+0x1a/0x1c0
CPU: 1 PID: 1881 Comm: cat Not tainted 4.14.0-rc2.blk_mq_io_hang+ #516
Call Trace:
 __scsi_format_command+0x27/0xc0
 scsi_show_rq+0x5c/0xc0
 __blk_mq_debugfs_rq_show+0x116/0x130
 blk_mq_debugfs_rq_show+0xe/0x10
 seq_read+0xfe/0x3b0
 full_proxy_read+0x54/0x90
 __vfs_read+0x37/0x160
 vfs_read+0x96/0x130
 SyS_read+0x55/0xc0
 entry_SYSCALL_64_fastpath+0x1a/0xa5

[mkp: added Type 2]

Fixes: 0eebd005 ("scsi: Implement blk_mq_ops.show_rq()")
Reported-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: James E.J. Bottomley <jejb@linux.vnet.ibm.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>