PM / Sleep: JBD and JBD2 missing set_freezable()

With the latest and greatest changes to the freezer, I started seeing
panics that were caused by jbd2 running post-process freezing and
hitting the canary BUG_ON for non-TuxOnIce I/O submission. I've traced
this back to a lack of set_freezable calls in both jbd and jbd2. Since
they're clearly meant to be frozen (there are tests for freezing()), I
submit the following patch to add the missing calls.

Signed-off-by: Nigel Cunningham <nigel@tuxonice.net>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>