x86: Reduce verbosity of "TSC is reliable" message

On modern systems, the kernel prints the message

    Skipping synchronization checks as TSC is reliable.

once for every non-boot CPU.

This gets kind of ridiculous on huge systems; for example, on a
64-thread system I was lucky enough to get:

    $ dmesg | grep 'TSC is reliable' | wc
         63     567    4221

There's no point to doing this for every CPU, since the code is
just checking the boot CPU anyway, so change this to a
printk_once() to make the message appears only once.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
LKML-Reference: <adazl8l2swc.fsf@cisco.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>