After ARM supports p4d page tables, the pg_level for note_page()
in walk_pmd() should be 4, not 3, fix it.

Fixes: 84e6ffb2 ("arm: add support for folded p4d page tables")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>