malloc: grow the padding for the malloc header

It was only two longs, and that's not working on riscv.

This whole malloc needs a complete makeover.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>

sys/src/libc/port/malloc.c

@@ -199,7 +199,7 @@ ppanic(Pool *p, char *fmt, ...)
 
 /* tracing */
 enum {
-	Npadlong	= 2,
+	Npadlong	= 8,
 	MallocOffset = 0,
 	ReallocOffset = 1
 };