scsi: lpfc: Fix silent memory allocation failure in lpfc_sli4_bsg_link_diag_test()

In the unlikely case of a failure to allocate an LPFC_MBOXQ_t structure, no
return status is set, thus the routine never logs an error and returns
success to the callee.

Fix by setting a return code on failure.

Link: https://lore.kernel.org/r/20210412013127.2387-9-jsmart2021@gmail.com


Co-developed-by: Justin Tee <justin.tee@broadcom.com>
Signed-off-by: Justin Tee <justin.tee@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>