dm vdo uds-threads: eliminate uds_*_semaphore interfaces

The implementation of thread 'barrier' data structure does not require
overdone private semaphore wrappers.  Also rename the barrier
structure's 'mutex' member (a semaphore) to 'lock'.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>