guard against bogus 0-extension of masks.  most of these are harmless to real
file servers now, as the largest qid is just 24-bits, but as this number increases
it will be more imporant.  the real fix is to mmu.c, which has a real bug in PPN.

Reference: /n/atom/patch/applied/fssignextend
Date: Sat Aug 15 23:12:56 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/fs/port/iobuf.c	Sat Aug 15 23:11:54 2015
+++ /sys/src/fs/port/iobuf.c	Sat Aug 15 23:11:55 2015
@@ -227,7 +227,7 @@
 		return 2;
 	}
 	if(qpath != QPNONE) {
-		if((qpath ^ t->path) & ~QPDIR) {
+		if((qpath ^ t->path) & ~(uvlong)QPDIR) {
 			print("	tag/path = %llud; expected %d/%llux\n",
 				(Wideoff)t->path, tag, (Wideoff)qpath);
 			return 0;
--- /sys/src/fs/port/chk.c	Sat Aug 15 23:11:57 2015
+++ /sys/src/fs/port/chk.c	Sat Aug 15 23:11:59 2015
@@ -509,7 +509,7 @@
 	}
 
 	/* check qid */
-	edent.qpath = d->qid.path & ~QPDIR;
+	edent.qpath = d->qid.path & ~(uvlong)QPDIR;
 	qmark(edent.qpath);
 	if(edent.qpath > maxq)
 		maxq = edent.qpath;
--- /sys/src/fs/port/9p2.c	Sat Aug 15 23:12:02 2015
+++ /sys/src/fs/port/9p2.c	Sat Aug 15 23:12:04 2015
@@ -223,7 +223,7 @@
 void
 mkqid9p2(Qid* qid, Qid9p1* qid9p1, int mode9p1)
 {
-	qid->path = (ulong)(qid9p1->path & ~QPDIR);
+	qid->path = (ulong)(qid9p1->path & ~(uvlong)QPDIR);
 	qid->vers = qid9p1->version;
 	qid->type = mktype9p2(mode9p1);
 }
--- /sys/src/fs/amd64/mmu.c	Sat Aug 15 23:12:06 2015
+++ /sys/src/fs/amd64/mmu.c	Sat Aug 15 23:12:08 2015
@@ -51,7 +51,7 @@
  *	mmuptcopy (PteSHARED trick?);
  */
 
-#define PPN(x)		((x)&~(PGSZ-1))
+#define PPN(x)		((x)&~(uintptr)(PGSZ-1))
 
 /*
  * set up a pat mappings.  the system depends