blkcg, writeback: dead memcgs shouldn't contribute to writeback ownership arbitration

wbc_account_io() collects information on cgroup ownership of writeback
pages to determine which cgroup should own the inode.  Pages can stay
associated with dead memcgs but we want to avoid attributing IOs to
dead blkcgs as much as possible as the association is likely to be
stale.  However, currently, pages associated with dead memcgs
contribute to the accounting delaying and/or confusing the
arbitration.

Fix it by ignoring pages associated with dead memcgs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>