fix possible memory leak in deallocuvm

when a zero PDE is encountered while searching for present PTEs to free, resume searching at first entry of the next page table instead of the current entry of the next page table.

fix possible memory leak in deallocuvm
ffe44492 · Cody Cutler · 91fd3470 · ffe44492
--- a/vm.c
+++ b/vm.c
@@ -266,7 +266,7 @@ deallocuvm(pde_t *pgdir, uint oldsz, uint newsz)
  for(; a  < oldsz; a += PGSIZE){
    pte = walkpgdir(pgdir, (char*)a, 0);
    if(!pte)
-      a += (NPTENTRIES - 1) * PGSIZE;
+      a = PGADDR(PDX(a) + 1, 0, 0) - PGSIZE;
    else if((*pte & PTE_P) != 0){
      pa = PTE_ADDR(*pte);
      if(pa == 0)