dm vdo: tweak wait_for_completion_interruptible callers

Update uds_join_threads to delay in wait_for_completion_interruptible
loop. And cleanup style nits in perform_admin_operation().

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