jbd: fix race in buffer processing in commit code

commit a61d90d75d0f9e86432c45b496b4b0fbf0fd03dc upstream.

In commit code, we scan buffers attached to a transaction.  During this
scan, we sometimes have to drop j_list_lock and then we recheck whether
the journal buffer head didn't get freed by journal_try_to_free_buffers().
 But checking for buffer_jbd(bh) isn't enough because a new journal head
could get attached to our buffer head.  So add a check whether the journal
head remained the same and whether it's still at the same transaction and
list.

This is a nasty bug and can cause problems like memory corruption (use after
free) or trigger various assertions in JBD code (observed).

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: <linux-ext4@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>