dm bufio: add lock_history optimization for cache iterators

Sometimes it is beneficial to repeatedly get and drop locks as part of
an iteration.  Introduce lock_history struct to help avoid redundant
drop and gets of the same lock.

Optimizes cache_iterate, cache_mark_many and cache_evict.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>