ext3: Add sanity check to make_indexed_dir

commit a21102b55c4f8dfd3adb4a15a34cd62237b46039 upstream.

Make sure the rec_len field in the '..' entry is sane, lest we overrun
the directory block and cause a kernel oops on a purposefully
corrupted filesystem.

This fixes a bug related to a bug originally reported by Sami Liedes
for ext4 at:

http://bugzilla.kernel.org/show_bug.cgi?id=12430

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>