liblog: BM_log_latency improvement

This reduces BM_log_latency from ~300ns to ~100ns.  Because, well, we
read the clock earlier and that is what BM_log_latency tries to
measure.  The one major improvement in the switch from kernel logger
to user space logger was that we picked up the timestamp in the
context of the caller before doing anything else, but alas changes
over time neglected this fact and placed isloggable checking ahead
of this important fact.

Test: liblog_benchmarks, check results
Change-Id: I4bc9fc3cf8b1659e88417d967b1d0f3743f9e456