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>