kmod: build with INVARIANTS for clang-analyzer

[kevans]

Summary

INVARIANTS sometimes informally documents some of the less obvious
assumptions that trip up clang --analyze with a path into a __dead2
function. Turning it on doesn't affect much else, so let's do it.

[mjg]

This is going to hide a host of write-only or straight up unused vars when the kernel is built without invariants.

If anything the thing to do here is to rework all the assert macros to expand to __builtin_unreachable if the asserted condition does not hold. However, that's quite a churn.

[kevans]

In D32861#741789, @mjg wrote:

This is going to hide a host of write-only or straight up unused vars when the kernel is built without invariants.

If anything the thing to do here is to rework all the assert macros to expand to __builtin_unreachable if the asserted condition does not hold. However, that's quite a churn.

[Phabricator is magic]. I don't see how you intend to do that, either, if the very act of observing the write-only var is enough to kill it.