scsi: libfc: Don't have fc_exch_find log errors on a new exchange

With the error message I added in "libfc: sanity check cpu number
extracted from xid" I didn't account for the fact that fc_exch_find is
called with FC_XID_UNKNOWN at the start of a new exchange if we are the
responder.

It doesn't come up with the initiator much, but that's basically every
exchange for a target.  By checking the xid for FC_XID_UNKNOWN first, we
not only prevent the erroneous error message, but skip the unnecessary
lookup attempt as well.

[mkp: applied by hand due to conflict with Hannes' libfc patch series]
Signed-off-by: Chris Leech <cleech@redhat.com>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libfc/fc_exch.c

@@ -939,6 +939,9 @@ static struct fc_exch *fc_exch_find(struct fc_exch_mgr *mp, u16 xid)
 	struct fc_exch *ep = NULL;
 	u16 cpu = xid & fc_cpu_mask;
 
+	if (xid == FC_XID_UNKNOWN)
+		return NULL;
+
 	if (cpu >= nr_cpu_ids || !cpu_possible(cpu)) {
 		pr_err("host%u: lport %6.6x: xid %d invalid CPU %d\n:",
 		       lport->host->host_no, lport->port_id, xid, cpu);