NVMe: Remove check for null

Checking fails static analysis due to additional arithmetic prior to
the NULL check. Mapping doesn't return NULL here anyway, so removing
the check.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>