use m->proc not up because we may be called from the clock interrupt.

Reference: /n/atom/patch/applied/mmuswitch
Date: Wed Feb 26 02:50:53 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/mmu.c	Wed Feb 26 02:50:27 2014
+++ /sys/src/nix/k10/mmu.c	Wed Feb 26 02:50:29 2014
@@ -104,8 +104,8 @@
 	Mpl pl;
 
 	pl = splhi();
-	up->newtlb = 1;
-	mmuswitch(up);
+	m->proc->newtlb = 1;
+	mmuswitch(m->proc);
 	splx(pl);
 }