MDEV-22871 follow-up: Align buf_pool.page_hash.array

buf_pool_t::page_hash_table::create(): Use aligned_malloc()
for the hash array, to get the page_hash_latch aligned on the
same L1 data cache line as the data that it is protecting.

storage/innobase/buf/buf0buf.cc

@@ -1488,8 +1488,10 @@ static void buf_block_free_mutexes(buf_block_t* block)
 void buf_pool_t::page_hash_table::create(ulint n)
 {
   n_cells= ut_find_prime(n);
-  array= static_cast<hash_cell_t*>
-    (ut_zalloc_nokey(pad(n_cells) * sizeof *array));
+  const size_t size= pad(n_cells) * sizeof *array;
+  void* v= aligned_malloc(size, CPU_LEVEL1_DCACHE_LINESIZE);
+  memset(v, 0, size);
+  array= static_cast<hash_cell_t*>(v);
 }
 
 /** Create the buffer pool.

storage/innobase/include/buf0buf.h

@@ -1832,7 +1832,7 @@ class buf_pool_t
 
     /** number of payload elements in array[] */
     Atomic_relaxed<ulint> n_cells;
-    /** the hash array, with pad(n_cells) elements */
+    /** the hash table, with pad(n_cells) elements, aligned to L1 cache size */
     hash_cell_t *array;
 
     /** Create the hash table.
@@ -1840,7 +1840,7 @@ class buf_pool_t
     void create(ulint n);
 
     /** Free the hash table. */
-    void free() { ut_free(array); array= nullptr; }
+    void free() { aligned_free(array); array= nullptr; }
 
     /** @return the index of an array element */
     ulint calc_hash(ulint fold) const { return calc_hash(fold, n_cells); }