Magazines must be freed under the expand lock (netty#14336)

Motivation:
It's unlikely, but possible, for our freeing of magazines to run
concurrently with magazine expansion. If that happens, we might not free
all magazines.

Modification:
Free the magazines under the expansion lock, and short-circuit the
magazine expansion if the allocator has been freed.

Result:
Fixes a data race that could leave magazines and their chunks un-freed.

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

@@ -292,7 +292,7 @@ private boolean tryExpandMagazines(int currentLength) {
         if (writeLock != 0) {
             try {
                 Magazine[] mags = magazines;
-                if (mags.length >= MAX_STRIPES || mags.length > currentLength) {
+                if (mags.length >= MAX_STRIPES || mags.length > currentLength || freed) {
                     return true;
                 }
                 int preferredChunkSize = mags[0].sharedPrefChunkSize;
@@ -335,8 +335,14 @@ protected void finalize() throws Throwable {
 
     private void free() {
         freed = true;
-        for (Magazine magazine : magazines) {
-            magazine.free();
+        long stamp = magazineExpandLock.writeLock();
+        try {
+            Magazine[] mags = magazines;
+            for (Magazine magazine : mags) {
+                magazine.free();
+            }
+        } finally {
+            magazineExpandLock.unlockWrite(stamp);
         }
         for (;;) {
             Chunk chunk = centralQueue.poll();