mm: nommu: don't scan the vma list when deleting

Since commit 297c5eee ("mm: make the vma list be doubly linked") made
it a doubly linked list, we don't need to scan the list when deleting
@vma.

And the original code didn't update the prev pointer. Fix it too.
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/nommu.c

@@ -746,7 +746,6 @@ static void add_vma_to_mm(struct mm_struct *mm, struct vm_area_struct *vma)
  */
 static void delete_vma_from_mm(struct vm_area_struct *vma)
 {
-	struct vm_area_struct **pp;
 	struct address_space *mapping;
 	struct mm_struct *mm = vma->vm_mm;
 
@@ -769,12 +768,14 @@ static void delete_vma_from_mm(struct vm_area_struct *vma)
 
 	/* remove from the MM's tree and list */
 	rb_erase(&vma->vm_rb, &mm->mm_rb);
-	for (pp = &mm->mmap; *pp; pp = &(*pp)->vm_next) {
-		if (*pp == vma) {
-			*pp = vma->vm_next;
-			break;
-		}
-	}
+
+	if (vma->vm_prev)
+		vma->vm_prev->vm_next = vma->vm_next;
+	else
+		mm->mmap = vma->vm_next;
+
+	if (vma->vm_next)
+		vma->vm_next->vm_prev = vma->vm_prev;
 
 	vma->vm_mm = NULL;
 }