5045 use atomic_{inc,dec}_* instead of atomic_add_*

@@ -173,11 +173,11 @@
 
         mutex_enter(&db->db_mtx);
         db->db_hash_next = h->hash_table[idx];
         h->hash_table[idx] = db;
         mutex_exit(DBUF_HASH_MUTEX(h, idx));
-        atomic_add_64(&dbuf_hash_count, 1);
+        atomic_inc_64(&dbuf_hash_count);
 
         return (NULL);
 }
 
 /*

@@ -208,11 +208,11 @@
                 ASSERT(dbf != NULL);
         }
         *dbp = db->db_hash_next;
         db->db_hash_next = NULL;
         mutex_exit(DBUF_HASH_MUTEX(h, idx));
-        atomic_add_64(&dbuf_hash_count, -1);
+        atomic_dec_64(&dbuf_hash_count);
 }
 
 static arc_evict_func_t dbuf_do_evict;
 
 static void