[SCSI] libfc: don't do discovery before callback is set

It's possible to "restart" discovery before it was started if
an RSCN is received early enough.  We were jumping to 0
due to the disc_callback function pointer not getting set.

Don't restart discovery if disc_callback is NULL.
Signed-off-by: NJoe Eykholt <jeykholt@cisco.com>
Signed-off-by: NRobert Love <robert.w.love@intel.com>
Signed-off-by: NJames Bottomley <James.Bottomley@suse.de>

drivers/scsi/libfc/fc_disc.c

@@ -219,6 +219,9 @@ static void fc_disc_recv_req(struct fc_seq *sp, struct fc_frame *fp,
  */
 static void fc_disc_restart(struct fc_disc *disc)
 {
+	if (!disc->disc_callback)
+		return;
+
 	FC_DISC_DBG(disc, "Restarting discovery\n");
 
 	disc->requested = 1;