The null checks on nvmebuf are redundant as nvmebuf is always obtained from
a container_of() and hence can never be null. Remove all the redundant null
checks. This also cleans up a static analysis warning.

Detected by CoverityScan, CID#1471753 ("Dereference before null check")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>