mtd: tests: Replace timeval with ktime_t

Changes the 32-bit time type timeval to the 64-bit time type
ktime_t, since 32-bit systems using struct timeval will break in the
year 2038. Correspondingly change do_gettimeofday() to ktime_get()
since ktime_get returns a ktime_t, but do_gettimeofday returns a
struct timeval.Here, ktime_get() is used instead of ktime_get_real()
since ktime_get() uses monotonic clock.

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>