libfrog: split workqueue destroy functions

libfrog: split workqueue destroy functions

Split the workqueue destroy function into two parts -- one to signal all
the threads to exit and wait for them, and a second one that actually
destroys all the memory associated with the workqueue.  This mean we can
report latent workqueue errors independent of the freeing function.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>