ipv6: Use ipv6_get_dsfield() instead of ipv6_tclass().

Commit 7a3198a8 ("ipv6: helper function to get tclass") introduced
ipv6_tclass(), but similar function is already available as
ipv6_get_dsfield().

We might be able to call ipv6_tclass() from ipv6_get_dsfield(),
but it is confusing to have two versions.

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