dm mpath: enable WRITE SAME support

Enable WRITE SAME support in dm multipath.  As far as multipath is
concerned it is just another write request.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Tested-by: Bharata B Rao <bharata.rao@gmail.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-mpath.c

@@ -907,6 +907,7 @@ static int multipath_ctr(struct dm_target *ti, unsigned int argc,
 
 	ti->num_flush_bios = 1;
 	ti->num_discard_bios = 1;
+	ti->num_write_same_bios = 1;
 
 	return 0;