aufs proc_map: support for /proc/PID/map_files and CONFIG_CHECKPOINT_RESTORE

Under the /proc/PID/map_files dir, there are symlinks pointing file
paths. Those path should be aufs's instead of the branch fs's.

Reported-by: Andrey Wagin <avagin@gmail.com>
See-also: http://lists.openvz.org/pipermail/criu/2014-September/016305.html
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
Merge branch 'aufs3.9/11proc_map' into aufs3.9/18public
aufs: support for AT_EMPTY_PATH/LOOKUP_EMPTY

Support the case like this.
	fd = open(filename);
	unlink(filename);
	name_to_handle_at(fd, "", fh, &mnt_id, AT_EMPTY_PATH);

In other words, aufs_d_revalidate() should allow the unlinked name when
LOOKUP_EMPTY is set.

Reported-by: Andrey Wagin <avagin@gmail.com>
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
Merge remote-tracking branch 'aufs3-standalone/aufs3.9/00base' into aufs3.9/10static
Merge branch 'aufs3.9/10static' into aufs3.9/11proc_map
Merge branch 'aufs3.9/11proc_map' into aufs3.9/18public