Hi all,

This is a reincarnation of the task->mm fixes. Several architectures
were traverse the tasklist in an unsafe manner, plus there are a
few cases of unsafe access to task->mm.

In v2 I decided to introduce a small helper in cpu.c: most arches
duplicate the same [buggy] code snippet, so it's better to fix it
and move the logic into a common function.

Thanks!

-- 
Anton Vorontsov
Email: cbouatmailru@gmail.com