workqueue: bound maximum queue depth

workqueue: bound maximum queue depth

Existing users of workqueues have bound maximum queue depths in
their external algorithms (e.g. prefetch counts). For parallelising
work that doesn't have an external bound, allow workqueues to
throttle incoming requests at a maximum bound. Bounded workqueues
also need to distribute work over all worker threads themselves as
there is no external bounding or worker function throttling
provided.

Existing callers are not throttled and retain direct control of
worker threads, only users of the new create interface will be
throttled and concurrency managed.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>