[SCSI] zfcp: fix race conditions when accessing erp_action lists

o always use locking when changing erp_action lists,
o avoid escalation to ERP_ACTION_REOPEN_PORT_FORCED if erp_action is
  still in use for ERP_ACTION_REOPEN_PORT

Signed-off-by: Andreas Herrmann <aherrman@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>