ext4: fix buffer leak in __ext4_read_dirblock() on error path

commit de59fae0 upstream.

Fixes: dc6982ff ("ext4: refactor code to read directory blocks ...")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org # 3.9
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>