scsi: lpfc: Fix Oops in nvme_register with target logout/login

lpfc_nvme_register_port hit a null prev_ndlp pointer in a test with lots of
target ports swapping addresses. The oldport value was stale, thus it's
ndlp (prev_ndlp set to it) was used.

Fix by moving oldrport pointer checks, and if used prev_ndlp pointer
assignment, to be done while the lock is held.

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>