block: sx8.c: Replace timeval with ktime_t

32-bit systems using 'struct timeval' will break in the year 2038,
in order to avoid that replace the code with more appropriate types.
This patch replaces timeval with 64 bit ktime_t which is y2038 safe.
Since st->timestamp is only interested in seconds, directly using
time64_t here. Function ktime_get_seconds is used since it uses
monotonic instead of real time and thus will not cause overflow.

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>