move vn_iowait / vn_iowake into xfs_aops.c

The whole machinery to wait on I/O completion is related to the I/O path
and should be there instead of in xfs_vnode.c.  Also give the functions
more descriptive names.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>