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pc64: fix for unsigned comparsion of (top - base) >= size

the rounding of base can make it above top, so have to
use signed comparsion.

--- a/sys/src/9/pc64/main.c

+++ b/sys/src/9/pc64/main.c

@@ -278,7 +278,7 @@

 		pm = &palloc.mem[i];

 		base = ROUND(pm->base, PGLSZ(1));

 		top = pm->base + (uvlong)pm->npage * BY2PG;

-		if((base + size) <= VMAPSIZE && (top - base) >= size){

+		if((base + size) <= VMAPSIZE && (vlong)(top - base) >= size){

 			va = base + VMAP;

 			pmap(m->pml4, base | PTEGLOBAL|PTEWRITE|PTEVALID, va, size);

 			palloc.pages = (Page*)va;