fs/userfaultfd: Fix maple tree iterator in userfaultfd_unregister()

When iterating the VMAs, the maple state needs to be invalidated if the
tree is modified by a split or merge to ensure the maple tree node
contained in the maple state is still valid.  These invalidations were
missed, so add them to the paths which alter the tree.

Reported-by: syzbot+0d2014e4da2ccced5b41@syzkaller.appspotmail.com
Fixes: 69dbe6da (userfaultfd: use maple tree iterator to iterate VMAs)
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -1630,17 +1630,20 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 				 NULL_VM_UFFD_CTX, anon_vma_name(vma));
 		if (prev) {
 			vma = prev;
+			mas_pause(&mas);
 			goto next;
 		}
 		if (vma->vm_start < start) {
 			ret = split_vma(mm, vma, start, 1);
 			if (ret)
 				break;
+			mas_pause(&mas);
 		}
 		if (vma->vm_end > end) {
 			ret = split_vma(mm, vma, end, 0);
 			if (ret)
 				break;
+			mas_pause(&mas);
 		}
 	next:
 		/*