KVM: arm64: Fix __pkvm_host_mkyoung_guest() return value

Don't use an uninitialised stack variable, and just return 0
on the non-error path.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502100911.8c9DbtKD-lkp@intel.com/


Reviewed-by: Quentin Perret <qperret@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>