Linux 5.6 compat: ktime_get_raw_ts64()

The getrawmonotonic() and getrawmonotonic64() interfaces have been
fully retired. Update gethrtime() to use the replacement interface
ktime_get_raw_ts64() which was introduced in the 4.18 kernel.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10052
Closes #10064