[SCSI] libfc: add print of exchange id for debugging fc_fcp

This is very helpful to match up the corresponding exchange to the actual I/O
described by the fsp, particularly when you do a side-by-side comparison of
the syslog with your trace.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>