Use reduced precision for scan times

Scan time limits do not need precision beyond 1ms. Switching
scn_sync_start_time and spa_sync_starttime from gethrtime() to
getlrtime() saves ~3% of CPU time during resilver scan stage.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18061