parisc: fix expand_stack() conversion

In commit 8d7071af89 ("mm: always expand the stack with the mmap write
lock held") I tried to deal with the remaining odd page fault handling
cases.  The oddest one is ia64, which has stacks that grow both up and
down.  And because ia64 was _so_ odd, I asked people to verify the end
result.

But a close second oddity is parisc, which is the only one that has a
main stack growing up (our "CONFIG_STACK_GROWSUP" config option).  But
it looked obvious enough that I didn't worry about it.

I should have worried a bit more.  Not because it was particularly
complex, but because I just used the wrong variable name.

The previous vma isn't called "prev", it's called "prev_vma".  Blush.

Fixes: 8d7071af89 ("mm: always expand the stack with the mmap write lock held")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/parisc/mm/fault.c

@@ -289,7 +289,7 @@ retry:
 	mmap_read_lock(mm);
 	vma = find_vma_prev(mm, address, &prev_vma);
 	if (!vma || address < vma->vm_start) {
-		if (!prev || !(prev->vm_flags & VM_GROWSUP))
+		if (!prev_vma || !(prev_vma->vm_flags & VM_GROWSUP))
 			goto bad_area;
 		vma = expand_stack(mm, address);
 		if (!vma)