scsi: lpfc: Fix null pointer dereference in lpfc_prep_els_iocb()

In the Linux kernel, the following vulnerability has been resolved: scsi: lpfc: Fix null pointer dereference in lpfc_prep_els_iocb() 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.