kvm: x86: Adjust the location of pkru_mask of kvm_mmu to reduce memory

Adjust the field pkru_mask to the back of direct_map to make up 8-byte
alignment.This reduces the size of kvm_mmu by 8 bytes.

Signed-off-by: Peng Hao <flyingpeng@tencent.com>
Message-Id: <20220228030749.88353-1-flyingpeng@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>