mm: get rid of the most spurious find_vma_prev() users

We have some users of this function that date back to before the vma
list was doubly linked, and just are silly.  These days, you can find
the previous vma by just following the vma->vm_prev pointer.

In some cases you don't need any find_vma() lookup at all, and in other
cases you're better off with the regular "find_vma()" that uses the vma
cache front-end lookup.

Some "find_vma_prev()" users are still valid, though.  For example, in
the case of a stack that grows up, it can be the case that we don't find
any 'vma' at all (because we're looking up an address that is past the
last vma), and that the stack that we want to grow is the 'prev' vma.

But that kind of special case aside, we generally should prefer to use
'find_vma()'.

Noticed due to a totally unrelated POWER memory corruption bug that just
happened to hit in 'find_vma_prev()' and made me go "Hmm - why are we
using that function here?".
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -906,14 +906,7 @@ struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *vma)
 	if (anon_vma)
 		return anon_vma;
 try_prev:
-	/*
-	 * It is potentially slow to have to call find_vma_prev here.
-	 * But it's only on the first write fault on the vma, not
-	 * every time, and we could devise a way to avoid it later
-	 * (e.g. stash info in next's anon_vma_node when assigning
-	 * an anon_vma, or when trying vma_merge).  Another time.
-	 */
-	BUG_ON(find_vma_prev(vma->vm_mm, vma->vm_start, &near) != vma);
+	near = vma->vm_prev;
 	if (!near)
 		goto none;
 
@@ -2044,9 +2037,10 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len)
 		return -EINVAL;
 
 	/* Find the first overlapping VMA */
-	vma = find_vma_prev(mm, start, &prev);
+	vma = find_vma(mm, start);
 	if (!vma)
 		return 0;
+	prev = vma->vm_prev;
 	/* we have  start < vma->vm_end  */
 
 	/* if it doesn't overlap, we have nothing.. */