xfs: make xfs_buf_ioend_async() static

There are no callers of the xfs_buf_ioend_async() function outside
of the fs/xfs/xfs_buf.c. So, let's make it static.
Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_buf.c

@@ -1045,7 +1045,7 @@ xfs_buf_ioend_work(
 	xfs_buf_ioend(bp);
 }
 
-void
+static void
 xfs_buf_ioend_async(
 	struct xfs_buf	*bp)
 {