gup: avoid stack expansion warning for known-good case

In commit a425ac53 ("gup: add warning if some caller would seem to
want stack expansion") I added a temporary warning to catch any strange
GUP users that would be impacted by the fact that GUP no longer extends
the stack.

But it turns out that the warning is most easily triggered through
__access_remote_vm(), that already knows to expand the stack - it just
does it *after* calling GUP.  So the warning is easy to trigger by just
running gdb (or similar) and accessing things remotely under the stack.

This just adds a temporary extra "expand stack early" to avoid the
warning for the already converted case - not because the warning is bad,
but because getting the warning for this known good case would then hide
any subsequent warnings for any actually interesting cases.

Let's try to remember to revert this change when we remove the warnings.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -5694,6 +5694,10 @@ int __access_remote_vm(struct mm_struct *mm, unsigned long addr, void *buf,
 	if (mmap_read_lock_killable(mm))
 		return 0;
 
+	/* Avoid triggering the temporary warning in __get_user_pages */
+	if (!vma_lookup(mm, addr) && !expand_stack(mm, addr))
+		return 0;
+
 	/* ignore errors, just check how much was successfully transferred */
 	while (len) {
 		int bytes, offset;