dm vdo dedupe: silence sparse warnings about locking context imbalances

Annotate both open_index() and close_index() with
__must_hold(&zones->lock) to silence these sparse warnings:
 warning: context imbalance in 'close_index' - unexpected unlock
 warning: context imbalance in 'open_index' - 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>