Linux 4.20 compat: current_kernel_time()

Commit torvalds/linux@976516404 removed the current_kernel_time()
function (and several others). All callers are expected to use
current_kernel_time64(). Update the gethrestime_sec() wrapper
accordingly.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8074