ipv6: Optimize ipv6_change_dsfield().

Do not convert endian back and forth.
If the caller uses contant "mask" argument (and most callers do),
we can omit runtime endian conversion here.

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