lib: rt_names: Add rtnl_dsfield_get_name()

lib: rt_names: Add rtnl_dsfield_get_name()

For formatting DSCP (not full dsfield), it would be handy to be able to
just get the name from the name table, and not get any of the remaining
cruft related to formatting. Add a new entry point to just fetch the
name table string uninterpreted. Use it from rtnl_dsfield_n2a().

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@kernel.org>