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>