[IPV4]: Convert do_gettimeofday() to getnstimeofday().

What do_gettimeofday() does is to call getnstimeofday() and
to convert the result from timespec{} to timeval{}.
After that, these callers convert the result again to msec.
Use getnstimeofday() and convert the units at once.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>