xfs: remove xfs_wait_on_pages()

It's just a simple wrapper around a VFS function that is only called
by another function in xfs_fs_subr.c. Remove it and call the VFS
function directly.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Andrew Dahl <adahl@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>