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kernel: ignore last page at the top of virtual kernel address space for xalloc()

avoding kernel address -BY2PG because of end pointer wrapping to zero.

--- a/sys/src/9/port/xalloc.c

+++ b/sys/src/9/port/xalloc.c

@@ -71,8 +71,10 @@

 		/* first give to kernel */

 		if(n > 0){

 			m->kbase = (uintptr)KADDR(m->base);

-			m->klimit = (uintptr)KADDR(m->base+size-1)+1;

-			xhole(m->base, size);

+			m->klimit = (uintptr)m->kbase+size;

+			if(m->klimit == 0)

+				m->klimit = (uintptr)-BY2PG;

+			xhole(m->base, m->klimit - m->kbase);

 			kpages -= n;

 		}

 		/* if anything left over, give to user */