KVM: Explicitly use GFP_KERNEL_ACCOUNT for 'struct kvm_vcpu' allocations

Use GFP_KERNEL_ACCOUNT when allocating vCPUs to make it more obvious that
that the allocations are accounted, to make it easier to audit KVM's
allocations in the future, and to be consistent with other cache usage in
KVM.

When using SLAB/SLUB, this is a nop as the cache itself is created with
SLAB_ACCOUNT.

When using SLOB, there are caveats within caveats.  SLOB doesn't honor
SLAB_ACCOUNT, so passing GFP_KERNEL_ACCOUNT will result in vCPU
allocations now being accounted.   But, even that depends on internal
SLOB details as SLOB will only go to the page allocator when its cache is
depleted.  That just happens to be extremely likely for vCPUs because the
size of kvm_vcpu is larger than the a page for almost all combinations of
architecture and page size.  Whether or not the SLOB behavior is by
design is unknown; it's just as likely that no SLOB users care about
accounding and so no one has bothered to implemented support in SLOB.
Regardless, accounting vCPU allocations will not break SLOB+KVM+cgroup
users, if any exist.

Reviewed-by: Wanpeng Li <kernellwp@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210406190740.4055679-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>