cpuset_set{affinity,domain}: do not allow empty masks

[kevans]

Summary

cpuset_modify() would not currently catch this, because it only checks that the new mask is a subset of the root set and circumvents the EDEADLK check in cpuset_testupdate().

This change both directly validates the mask coming in since we can trivially detect an empty mask, and it updates cpuset_testupdate to catch stuff like this going forward by always ensuring we don't end up with an empty mask.

The check_mask argument has been renamed because the 'check' verbiage does not imply to me that it's actually doing a different operation. We're either augmenting the existing mask, or we are replacing it entirely.

Reported by: [e-mail]

Test Plan

I could not reproduce this one from syzbot's repro, but it's clear that it is a problem