x86/time: tsc_check_writability() may need to be run a second time

x86/time: tsc_check_writability() may need to be run a second time

While we shouldn't remove its current invocation, we need to re-run it
for the case that the X86_FEATURE_TSC_RELIABLE feature flag has been
cleared, in order to avoid using the TSC rendezvous function in case
the TSC can't be written.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>