dm: add dm_num_hash_locks()

Simple helper to use when DM core code needs to appropriately size,
based on num_online_cpus(), its data structures that split locks.

dm_num_hash_locks() rounds up num_online_cpus() to next power of 2
but caps return at DM_HASH_LOCKS_MAX (64).

This heuristic may evolve as warranted, but as-is it will serve as a
more informed basis for sizing the sharded lock structs in dm-bufio's
dm_buffer_cache (buffer_trees) and dm-bio-prison-v1's dm_bio_prison
(prison_regions).

Signed-off-by: Mike Snitzer <snitzer@kernel.org>