don't allow subtraction to fool us.

Reference: /n/atom/patch/applied2013/kseg2care
Date: Tue Sep 17 03:20:19 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/mmu.c	Tue Sep 17 03:20:02 2013
+++ /sys/src/nix/k10/mmu.c	Tue Sep 17 03:20:04 2013
@@ -575,6 +575,8 @@
 	int use, flags;
 	uintmem pa, sz;
 
+	if(va < KSEG2 || va > KSEG2END)
+		return -1;
 	pa = va - KSEG2;
 	if((pa = adrmemtype(pa, &sz, &flags, &use)) == 0 || use != Mvmap)
 		return -1;
--- /sys/src/nix/k10/mem.h	Tue Sep 17 03:20:06 2013
+++ /sys/src/nix/k10/mem.h	Tue Sep 17 03:20:07 2013
@@ -67,7 +67,7 @@
  *  Address spaces. Kernel, sorted by address.
  */
 #define KSEG2		(0xfffffe0000000000ull)	/* 1TB - KMAP */
-/*			 0xffffff0000000000ull	end of KSEG2 */
+#define KSEG2END	0xffffff0000000000ull	/* end of KSEG2 */
 #define KSEG0		(0xfffffffff0000000ull)	/* 256MB - this is confused */
 #define KZERO		(0xfffffffff0000000ull)
 #define KTZERO		(KZERO+1*MiB+64*KiB)