writeback: implement wb_wait_for_single_work()

For cgroup writeback, multiple wb_writeback_work items may need to be
issuedto accomplish a single task.  The previous patch updated the
waiting mechanism such that wb_wait_for_completion() can wait for
multiple work items.

Issuing mulitple work items involves memory allocation which may fail.
As most writeback operations can't fail or blocked on memory
allocation, in such cases, we'll fall back to sequential issuing of an
on-stack work item, which would need to be waited upon sequentially.

This patch implements wb_wait_for_single_work() which waits for a
single work item independently from wb_completion waiting so that such
fallback mechanism can be used without getting tangled with the usual
issuing / completion operation.

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