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>

drivers/md/dm.h

@@ -20,6 +20,7 @@
 #include <linux/completion.h>
 #include <linux/kobject.h>
 #include <linux/refcount.h>
+#include <linux/log2.h>
 
 #include "dm-stats.h"
 
@@ -228,4 +229,13 @@ void dm_free_md_mempools(struct dm_md_mempools *pools);
  */
 unsigned int dm_get_reserved_bio_based_ios(void);
 
+#define DM_HASH_LOCKS_MAX 64
+
+static inline unsigned int dm_num_hash_locks(void)
+{
+	unsigned int num_locks = roundup_pow_of_two(num_online_cpus());
+
+	return min_t(unsigned int, num_locks, DM_HASH_LOCKS_MAX);
+}
+
 #endif