dm vdo data-vio: silence sparse warnings about locking context imbalances

Factor wait_permit() out from acquire_permit() so that the latter
always holds the spinlock and the former always releases it.

Otherwise sparse complains about locking context imbalances due to
conditional spin_unlock in acquire_permit:
 warning: context imbalance in 'acquire_permit' - different lock contexts for basic block
 warning: context imbalance in 'vdo_launch_bio' - unexpected unlock

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>