scsi: fcoe: hold disc_mutex when traversing rport lists

When calling either fc_rport_logon() or fc_rport_logoff() during rport list
traversal we cannot use the RCU list traversal, as either of these
functions will be taking a mutex.  So we need to partially revert commit
a407c593 to take the disc mutex during traversal.  We should, however,
continue to use krefs to ensure that the rport object will not be freed
from under us.

Fixes: a407c593 ("scsi: libfc: Fixup disc_mutex handling")
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>