[NETFILTER]: x_tables: replace IPv4 DSCP target by address family independent version

This replaces IPv4 DSCP target by address family independent version.
This also
	- utilizes dsfield.h to get/mangle DS field in IPv4/IPv6 header
	- fixes Kconfig help text.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>