arm: revert MAXDSIZ change from 202aea9c82

This imposes a fairly severe limitation on space available for mmap that
was not noticed prior to commit. Unfixed mmap will only map from
[data + MAXSIZE, end of user VA space], bringing the amount of usable space
down way too low for non-trivial link jobs (for instance).

Reported by:	mmel

sys/arm/include/vmparam.h

@@ -50,7 +50,7 @@
 #define	DFLDSIZ		(128UL*1024*1024)	/* initial data size limit */
 #endif
 #ifndef	MAXDSIZ
-#define	MAXDSIZ		(1856UL*1024*1024)	/* max data size */
+#define	MAXDSIZ		(512UL*1024*1024)	/* max data size */
 #endif
 #ifndef	DFLSSIZ
 #define	DFLSSIZ		(4UL*1024*1024)		/* initial stack size limit */