utimensat: AT_EMPTY_PATH support

This makes it possible to use utimensat on an O_PATH file (including
symlinks).

It supersedes the nonstandard utimensat(fd, NULL, ...) form.

Cc: linux-api@vger.kernel.org
Cc: linux-man@vger.kernel.org
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>