kernel: work: Fix race in workqueue thread

After a call to k_work_flush returns the sync variable
may still be modified by the workq.  This is because
the work queue thread continues to modify the flag in
sync even after k_work_flush returns.  This commit adds
K_WORK_FLUSHING_BIT, and with this bit, we moved the
logic of waking up the caller from handle_flush to the
finalize_flush_locked in workq, so that after waking up
the caller, the workqueue will no longer operate on sync.

Fixes: #64530

Signed-off-by: Junfan Song <sjf221100@gmail.com>