Linux 4.20: current_kernel_time is gone

Linux 4.20: current_kernel_time is gone

With Linux commit 976516404ff3fab2a8caa8bd6f5efc1437fed0b8 'y2038:
remove unused time interfaces' (4.20-rc1), current_kernel_time() has
been removed.

Many y2038-compliant time APIs were introduced with Linux commit
fb7fcc96a86cfaef0f6dcc0665516aa68611e736 'timekeeping: Standardize on
ktime_get_*() naming' (4.18).  According to
Documentation/core-api/timekeeping.rst, a suitable replacement for:

  struct timespec current_kernel_time(void)

would be:

  void ktime_get_coarse_real_ts64(struct timespec64 *ts))

Add an autoconf test and equivalent logic to deal.

Change-Id: I4ff622ad40cc6d398267276d13493d819b877350
Reviewed-on: https://gerrit.openafs.org/13391
Tested-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>