Don't calculate max direct memory twice in PlatformDependent

Motivation:

I'm not sure if trivial changes like this are interesting :-) But I
noticed that the PlatformDependent.maxDirectMemory0() method is called
twice unnecessarily during static initialization (on the default path at
least).

Modifications:

Use constant MAX_DIRECT_MEMORY already set to the same value instead of
calling maxDirectMemory0() again.

Result:

A surely imperceivable reduction in operations performed at startup.

common/src/main/java/io/netty/util/internal/PlatformDependent.java

@@ -158,7 +158,7 @@ public final class PlatformDependent {
         } else {
             USE_DIRECT_BUFFER_NO_CLEANER = true;
             if (maxDirectMemory < 0) {
-                maxDirectMemory = maxDirectMemory0();
+                maxDirectMemory = MAX_DIRECT_MEMORY;
                 if (maxDirectMemory <= 0) {
                     DIRECT_MEMORY_COUNTER = null;
                 } else {