Null pointer dereference

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.