gettime: improve cpu clock test

gettime: improve cpu clock test

We're missing a synchronization before reading and comparing
with the current *t->seq in the loop. Rewrite the loop to
use compare-and-exchange for the increment, and ensure that
we have the proper synchronization inserted. This should both
be faster and more reliable.

Signed-off-by: Jens Axboe <axboe@kernel.dk>