scsi: lpfc: don't dereference dma_buf->iocbq before null check

From: Colin Ian King <colin.king@canonical.com>

dma_buf->iocbq is being dereferenced immediately before it is
being null checked, so we have a potential null pointer dereference
bug.  Fix this by only dereferencing it only once we have passed
a null check on the pointer.

Detected by CoverityScan, CID#1411652 ("Dereference before null check")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>