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>

fs/xfs/linux-2.6/xfs_super.c

@@ -1822,7 +1822,7 @@ init_xfs_fs(void)
 			 XFS_BUILD_OPTIONS " enabled\n");
 
 	ktrace_init(64);
-	vn_init();
+	xfs_ioend_init();
 	xfs_dir_startup();
 
 	error = xfs_init_zones();