isofs: Fix infinite looping over CE entries

isofs: Fix infinite looping over CE entries

Rock Ridge extensions define so called Continuation Entries (CE) which
define where is further space with Rock Ridge data. Corrupted isofs
image can contain arbitrarily long chain of these, including a one
containing loop and thus causing kernel to end in an infinite loop when
traversing these entries.

Limit the traversal to 32 entries which should be more than enough space
to store all the Rock Ridge data.

Change-Id: Ia0475fc07ee3e8ecc1d53673439f32c175acc10b
Reported-by: P J P <ppandit@redhat.com>
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>