[SCSI] libfc: export seq_release() for users of seq_assign()

Target modules using lport->tt.seq_assign() get a hold on the
exchange but have no way of releasing it.  Add that.

Signed-off-by: Joe Eykholt <jeykholt@cisco.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>