HP_MALLOC: Fine-tune the "frag split size" constants

It seems that "split on 4096 or larger" is too large a value, leading to
fragmentation issues on typical 1G - 4G SHM pools.

With this patch, HP_MALLOC will visibly use less shared memory, but will
also split fragments more often.

mem/hp_malloc.c

@@ -43,8 +43,8 @@
 #define MIN_FRAG_SIZE	ROUNDTO
 
 /* only perform a split if the resulting free fragment is at least this size */
-#define MIN_SHM_SPLIT_SIZE	4096
-#define MIN_PKG_SPLIT_SIZE	 256
+#define MIN_SHM_SPLIT_SIZE	 256
+#define MIN_PKG_SPLIT_SIZE	 128
 
 #define FRAG_NEXT(f) ((struct hp_frag *)(void *)((char *)((f) + 1) + (f)->size))