xfs: implement ->update_time

Use this new method to replace our hacky use of ->dirty_inode.  An additional
benefit is that we can now propagate errors up the stack.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>