scsi: libfc: Add WARN_ON() when deleting rports

The discovery rport list handling is quite odd; the list traversal is
independent from the lifetime of the rport itself. This makes auditing
quite tricky, and the chance remains that we've missed something.  So this
patch adds a WARN_ON() statement when freeing an rport which is still part
of a list.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>