NVMe: Free cmdid on nvme_submit_bio error

nvme_map_bio() is called after the cmdid is allocated, so we have to
free the cmdid before returning from nvme_submit_bio() if nvme_map_bio()
returned an error.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>