use the right hashpower for the item_locks table

For some reason I can't understand, I was accessing item_locks via the main
hash table's power level, then modulus'ed to the number of item locks.
hashpower can change as the hash table grows, except it only ever changes
while no item locks are being held (via the item_global_lock synchronization
bits). The item_locks hashpower is static for the duration.

So this isn't a safety issue, but instead just using the hash table wrong
and doing an extra modulus.

As an aside, this does improve benchmarks by a tiny bit.

Author: dormando

thread.c

@@ -56,6 +56,7 @@ static pthread_mutex_t cqi_freelist_lock;
 static pthread_mutex_t *item_locks;
 /* size of the item lock hash table */
 static uint32_t item_lock_count;
+static unsigned int item_lock_hashpower;
 #define hashsize(n) ((unsigned long int)1<<(n))
 #define hashmask(n) (hashsize(n)-1)
 /* this lock is temporarily engaged during a hash table expansion */
@@ -123,7 +124,7 @@ void item_unlock_global(void) {
 void item_lock(uint32_t hv) {
     uint8_t *lock_type = pthread_getspecific(item_lock_type_key);
     if (likely(*lock_type == ITEM_LOCK_GRANULAR)) {
-        mutex_lock(&item_locks[(hv & hashmask(hashpower)) % item_lock_count]);
+        mutex_lock(&item_locks[hv & hashmask(item_lock_hashpower)]);
     } else {
         mutex_lock(&item_global_lock);
     }
@@ -137,7 +138,7 @@ void item_lock(uint32_t hv) {
  * switch so it should stay safe.
  */
 void *item_trylock(uint32_t hv) {
-    pthread_mutex_t *lock = &item_locks[(hv & hashmask(hashpower)) % item_lock_count];
+    pthread_mutex_t *lock = &item_locks[hv & hashmask(item_lock_hashpower)];
     if (pthread_mutex_trylock(lock) == 0) {
         return lock;
     }
@@ -151,7 +152,7 @@ void item_trylock_unlock(void *lock) {
 void item_unlock(uint32_t hv) {
     uint8_t *lock_type = pthread_getspecific(item_lock_type_key);
     if (likely(*lock_type == ITEM_LOCK_GRANULAR)) {
-        mutex_unlock(&item_locks[(hv & hashmask(hashpower)) % item_lock_count]);
+        mutex_unlock(&item_locks[hv & hashmask(item_lock_hashpower)]);
     } else {
         mutex_unlock(&item_global_lock);
     }
@@ -803,6 +804,7 @@ void thread_init(int nthreads, struct event_base *main_base) {
     }
 
     item_lock_count = hashsize(power);
+    item_lock_hashpower = power;
 
     item_locks = calloc(item_lock_count, sizeof(pthread_mutex_t));
     if (! item_locks) {