Move gethrtime() calls out of vdev queue lock

This dramatically reduces the lock contention on systems with slower
(non-TSC) timecounters. With TSC the difference is minimal, but since
this lock is pretty congested, any improvement counts. Plus I don't
see any reason to do it under the lock other than the latency of the
lock itself, which this change actually reduces.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12281