[RFC][PATCH 0/7] kref improvements

[Peter Zijlstra]

This series unfscks kref and then implements it in terms of refcount_t.

x86_64-allyesconfig compile tested and boot tested with my regular config.

refcount_t is as per the previous thread, it BUGs on over-/underflow and
saturates at UINT_MAX, such that if we ever overflow, we'll never free again.