kill-the-bkl/reiserfs: panic in case of lock imbalance

Until now, trying to unlock the reiserfs write lock whereas the current
task doesn't hold it lead to a simple warning.
We should actually warn and panic in this case to avoid the user datas
to reach an unstable state.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Cc: Laurent Riffard <laurent.riffard@free.fr>