This patch improves ext3's ability to deal with corruption on-disk.  If we
try to delete a metadata block twice, we confuse ext3's internal revoke
error-checking, resulting in a BUG().  But this can occur in practice due
to a corrupt indirect block, so we should attempt to fail gracefully.

Downgrade the assert failure to a JH_EXPECT_BH failure, and return EIO when
it occurs.

This is easily reproduced with a sample ext3 fs image containing an inode
which references the same indirect block more than once.  Deleting that
inode will BUG() an unfixed kernel with:

Assertion failure in journal_revoke() at fs/jbd/revoke.c:379:
"!buffer_revoked(bh)"

With the fix, ext3 recovers gracefully.
Signed-off-by: Stephen Tweedie <sct@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>