LU-9019 zfs: use 64-bit timestamps for brw stats

LU-9019 zfs: use 64-bit timestamps for brw stats

The debugfs brw stats interface contains timestamps that are
computed from timeval, which overflows in 2038 on 32-bit systems.

This changes the output to use a timespec64 type to avoid the
overflow. I also change the format to print the sub-second portion
as 9 digits (nanoseconds) for clarity, rather than printing six
digits without leading zeroes.

Change-Id: I34f3b02a0d5f80e9ae22a47f55445199332b3dc9
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/25411
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>