add malloc_usable_size for libc malloc

this reduces the extra 8 bytes we save before each pointer.
but more importantly maybe, it makes the valgrind runs to be more similiar
to our normal runs.

note: the change in malloc_stats struct in server.h is to eliminate an name conflict.
structs that are not typedefed are resolved from a separate name space.

src/zmalloc.h

@@ -63,6 +63,11 @@
 
 #ifndef ZMALLOC_LIB
 #define ZMALLOC_LIB "libc"
+#ifdef __GLIBC__
+#include <malloc.h>
+#define HAVE_MALLOC_SIZE 1
+#define zmalloc_size(p) malloc_usable_size(p)
+#endif
 #endif
 
 /* We can enable the Redis defrag capabilities only if we are using Jemalloc