KVM: arm64: vgic-v3: Use u32 to manage the line level from userspace

Despite the userspace ABI clearly defining the bits dealt with by
KVM_DEV_ARM_VGIC_GRP_LEVEL_INFO as a __u32, the kernel uses a u64.

Use a u32 to match the userspace ABI, which will subsequently lead
to some simplifications.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>