xfs/279: use MKFS_XFS_PROG, not hardcoded path

I'm really not sure why this wasn't this way from the start.
On some systems it's in /usr/sbin, and the test fails.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/xfs/279

@@ -62,7 +62,7 @@ _check_mkfs()
 {
 	echo "===================" 
 	echo "mkfs with opts: $@" | sed -e "s,/dev/sd.,DEVICE,"
-	/sbin/mkfs.xfs $@ 2>/dev/null > $tmp.mkfs.full
+	$MKFS_XFS_PROG $@ 2>/dev/null > $tmp.mkfs.full
 	if [ $? -ne 0 ]; then
 		echo "Failed."
 		return