ext4: Add sanity check to make_indexed_dir

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.

Thanks to Sami Liedes for reporting this bug.

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

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org