vm: fix vm_pgoff wrap in stack expansion

Commit 982134ba ("mm: avoid wrapping vm_pgoff in mremap()") fixed
the case of a expanding mapping causing vm_pgoff wrapping when you used
mremap.  But there was another case where we expand mappings hiding in
plain sight: the automatic stack expansion.

This fixes that case too.

This one also found by Robert Święcki, using his nasty system call
fuzzer tool.  Good job.
Reported-and-tested-by: Robert Święcki <robert@swiecki.net>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1814,6 +1814,8 @@ static int expand_downwards(struct vm_area_struct *vma,
 		size = vma->vm_end - address;
 		grow = (vma->vm_start - address) >> PAGE_SHIFT;
 
+		error = -ENOMEM;
+		if (grow <= vma->vm_pgoff) {
 			error = acct_stack_growth(vma, size, grow);
 			if (!error) {
 				vma->vm_start = address;
@@ -1821,6 +1823,7 @@ static int expand_downwards(struct vm_area_struct *vma,
 				perf_event_mmap(vma);
 			}
 		}
+	}
 	vma_unlock_anon_vma(vma);
 	khugepaged_enter_vma_merge(vma);
 	return error;