backport commit ("739f79fc9db1 mm: memcontrol: fix NULL pointer crash in test_clear_page_writeback()") to linux-4.9-stable

Hi reviewers,

I suggest to backport commit "739f79fc9db1 mm: memcontrol: fix NULL pointer crash in test_clear_page_writeback()" to linux-4.9 stable tree.

This email reports a NULL pointer crash in test_clear_page_writeback() in android common kernel-4.9. There is a fix ("739f79fc9db1 mm: memcontrol: fix NULL pointer crash in test_clear_page_writeback()") in kernel-4.13.

commit: 739f79fc9db1b38f96b5a5109b247a650fbebf6d subject: mm: memcontrol: fix NULL pointer crash in test_clear_page_writeback() kernel version to apply to: Linux-4.9

cheers, Miles