Modify lockstat_nsecs() to just return unless lockstat probes are actually
enabled. The cost of a timecounter read can be quite significant, and the
problem became more apparent after r284297, since that change resulted in
a call to lockstat_nsecs() for each acquisition of an rwlock read lock.

PR: 201642
Reviewed by: avg
Tested by: Jason Unovitch
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D3073