[X/B/D/E/F] CVE-2020-11565

[Thadeu Lima de Souza Cascardo]

This was built and tested on Xenial 4.4 kernel.

[Impact]
An invalid mpol option when mounting a tmpfs might cause a crash.

[Test case]
mount -t tmpfs -o mpol=prefer:, none /mnt
Printint out the options, usually by cat /proc/self/mountinfo, is needed
to cause the crash.

[Regression potential]
Will cause mounting to fail when it could cause a crash before. So,
hardly worse.