writeback, memcg: Implement cgroup_writeback_by_id()

Implement cgroup_writeback_by_id() which initiates cgroup writeback
from bdi and memcg IDs.  This will be used by memcg foreign inode
flushing.

v2: Use wb_get_lookup() instead of wb_get_create() to avoid creating
    spurious wbs.

v3: Interpret 0 @nr as 1.25 * nr_dirty to implement best-effort
    flushing while avoding possible livelocks.

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