Block devices do not, in general, report congestion any more, so this
congestion_wait() is effectively just a sleep.

It isn't entirely clear what is being waited for, but as we only wait
when j_async_throttle is elevated, it seems reasonable to stop waiting
when j_async_throttle becomes zero - or after the same timeout.

So change to use wait_event_event_timeout() for waiting, and
wake_up_var() to signal an end to waiting.

Link: https://lore.kernel.org/r/163712368225.13692.3419908086400748349@noble.neil.brown.nameSigned-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jan Kara <jack@suse.cz>