Staging: fbtbt: Replace timespec with ktime_t

struct timespec will overflow in year 2038, so replace it with
ktime_t. And replace functions that use struct timespec,
timespec_sub with ktime_sub. Also use monotonic time instead of real
time, by replacing getnstimeofday with ktime_get, to be more robust
against leap seconds and settimeofday() calls.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>