Skip missing page directories in deallocuvm

Previously, deallocuvm scanned from 0 to KERNBASE in one page
increments, which had a noticable effect on boot time.  Now it skips
over missing page directories.

vm.c

@@ -261,7 +261,9 @@ deallocuvm(pde_t *pgdir, uint oldsz, uint newsz)
   a = PGROUNDUP(newsz);
   for(; a  < oldsz; a += PGSIZE){
     pte = walkpgdir(pgdir, (char*)a, 0);
-    if(pte && (*pte & PTE_P) != 0){
+    if(!pte)
+      a += (NPTENTRIES - 1) * PGSIZE;
+    else if((*pte & PTE_P) != 0){
       pa = PTE_ADDR(*pte);
       if(pa == 0)
         panic("kfree");