Fix few issues of LinuxKPI workqueue.

LinuxKPI workqueue wrappers reported "successful" cancellation for works
already completed in normal way. This change brings reported status and
real cancellation fact into sync. This required for drm-next operation.

Reviewed by: hselasky (earlier version)
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D11904