KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it

In the Linux kernel, the following vulnerability has been resolved:

KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it

Nullify have_run_cpus when freeing the mask, particularly in the error path
of __sev_guest_init(), so that KVM doesn't have to subtly use sev->active
to track whether or not the mask has been freed. As pointed out by Sashiko,
blindly freeing the mask in sev_vm_destroy() results in a double-free if
the mask is freed if __sev_guest_init() fails.

Throw the logic in a helper as nullifying the pointer is frustratingly
difficult and weird due to have_run_cpus being a single-entry array when
CPUMASK_OFFSTACK=n. Deliberately don't use CPUMASK_VAR_NULL, as it's not
directly assignable when the cpumask is on-stack, e.g. requires using a
local variable and a memcpy(), which is beyond ridiculous. Furthermore,
while clearing the on-stack bitmask is an unnecessary and arguably unwanted
side effect, KVM absolutely relies on '0' being the "null" value given that
the struct is zero-allocated.

Opportunistically add an alloc() helper to pair with free(); there are just
enough call sites to make doing so worthwhile.