mm/rmap: fix comment in anon_vma_clone()

Commit 2555283e ("mm/rmap: Fix anon_vma->degree ambiguity leading to
double-reuse") use num_children and num_active_vmas to replace the origin
degree to fix anon_vma UAF problem.  Update the comment in anon_vma_clone
to fit this change.

Link: https://lkml.kernel.org/r/20221014013931.1565969-1-mawupeng1@huawei.com


Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>