src: use UDATA defines from libnftnl

src: use UDATA defines from libnftnl

Userdata attribute names have been added to libnftnl, use them instead
of the local copy.

While being at it, rename udata_get_comment() in netlink_delinearize.c
and the callback it uses since the function is specific to rules. Also
integrate the existence check for NFTNL_RULE_USERDATA into it along with
the call to nftnl_rule_get_data().

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>