netlink_linearize: do not duplicate user data when linearizing user data

netlink_linearize: do not duplicate user data when linearizing user data

Otherwise, we memory leak this area since nftnl_rule_set_data() now
makes a copy of the user data which receives. This is happening since
libnftnl's ("rule: Fix segfault due to invalid free of rule user data"),
it is not necessary make a copy before call it.

Note: Carlos originally posted this patch under the name of ("nftables:
Fix memory leak linearizing user data").

Signed-off-by: Carlos Falgueras García <carlosfg@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>