KVM: selftests: Close opened file descriptor in stable_tsc_check_supported()

Close the "current_clocksource" file descriptor before returning or exiting
from stable_tsc_check_supported() in vmx_nested_tsc_scaling_test.

Signed-off-by: Hao Ge <gehao@kylinos.cn>
Reviewed-by: Vipin Sharma <vipinsh@google.com>
Link: https://lore.kernel.org/r/20230405101350.259000-1-gehao@kylinos.cn


Signed-off-by: Sean Christopherson <seanjc@google.com>