remove unnecessary vararg argument in PooledByteBufAllocator (#8338)

Motivation:

No need in varargs, the method always accepts array.

Modification:

... replaced with []

buffer/src/main/java/io/netty/buffer/PooledByteBufAllocator.java

@@ -580,7 +580,7 @@ public class PooledByteBufAllocator extends AbstractByteBufAllocator implements
         return usedMemory(directArenas);
     }
 
-    private static long usedMemory(PoolArena<?>... arenas) {
+    private static long usedMemory(PoolArena<?>[] arenas) {
         if (arenas == null) {
             return -1;
         }