fs: xfs needs inode_wait to be exported

Since wait_on_inode() references it.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>