KVM: arm64: Don't use __vcpu_sys_reg() to get the address of a sysreg

KVM: arm64: Don't use __vcpu_sys_reg() to get the address of a sysreg

We are about to prevent the use of __vcpu_sys_reg() as a lvalue,
and getting the address of a rvalue is not a thing.

Update the couple of places where we do this to use the __ctxt_sys_reg()
accessor, which return the address of a register.

Reviewed-by: Miguel Luis <miguel.luis@oracle.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250603070824.1192795-4-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>