fix regression in SCSI_IOCTL_SEND_COMMAND

blk_rq_set_block_pc() memsets rq->cmd to 0, so it should come
immediately after blk_get_request() to avoid overwriting the
user-supplied CDB.  Also check for failure to allocate rq.

Fixes: f27b087b81b7 ("block: add blk_rq_set_block_pc()")
Cc: <stable@vger.kernel.org> # 3.16.x
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: Jens Axboe <axboe@fb.com>