fix sign extension bug (thanks, charles!).  the proper fix to this is to
change PGSZ to be signed, but i wanted to get something in quickly.

Reference: /n/atom/patch/applied/mmusign
Date: Fri Aug 14 15:21:27 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/mmu.c	Fri Aug 14 15:20:47 2015
+++ /sys/src/nix/k10/mmu.c	Fri Aug 14 15:20:48 2015
@@ -14,7 +14,7 @@
  *	mmuptcopy (PteSHARED trick?);
  */
 
-#define PPN(x)		((x)&~(PGSZ-1))
+#define PPN(x)		((x)&~(uintmem)(PGSZ-1))
 
 /*
  * set up a pat mappings.  the system depends
@@ -692,7 +692,7 @@
  */
 typedef struct Remap Remap;
 struct Remap {
-	uintmem	map[300];
+	uintmem	map[600];
 	usize	n;
 };
 static	Remap	smap;
@@ -703,7 +703,8 @@
 	int i, o;
 	uintmem pa, p;
 
-	o = pa0 - (pa0 & ~(PGSZ-1));
+//	iprint("vmapoverlap(%#P, %#lux)\n", pa0, sz);
+	o = pa0 - (pa0 & ~(uintmem)(PGSZ-1));
 	pa = pa0 - o;
 	sz = ROUNDUP(sz+o, PGSZ);