LINUX: Update to Linux struct iattr->ia_ctime to timespec64 with 4.18

LINUX: Update to Linux struct iattr->ia_ctime to timespec64 with 4.18

With 4.18+ Linux kernels we see a transition to 64-bit time stamps by
default.

current_kernel_time() returns the 32-bit struct timespec.
current_kernel_time64() returns the 64-bit struct timespec64.

struct iattr->ia_ctime expects struct timespec64 as of 4.18+.

Timestamps greater than 31-bit rollover after 2147483647 or
January 19, 2038 03:14:07 UTC. This is the same approach taken by
the Linux developers for converting between timepsec64 and timespec.

Change-Id: Icc1cf5d1a6679f5c749f8720f225a9b293f675fd
Reviewed-on: https://gerrit.openafs.org/13241
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>