It is possible to call lpfc_issue_els_plogi() passing a did for which no
matching ndlp is found. A call is then made to lpfc_prep_els_iocb() with a
null pointer to a lpfc_nodelist structure resulting in a null pointer
dereference.

Fix by returning an error status if no valid ndlp is found. Fix up comments
regarding ndlp reference counting.

Link: https://lore.kernel.org/r/20210301171821.3427-10-jsmart2021@gmail.com
Fixes: 4430f7fd ("scsi: lpfc: Rework locations of ndlp reference taking")
Co-developed-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>