netfilter: nf_tables: fix userdata memleak

When userdata was introduced for tables and objects its allocation was
only freed inside the error path of the new{table, object} functions.

Free user data inside corresponding destroy functions for tables and
objects.

Fixes: b131c964 ("netfilter: nf_tables: add userdata support for nft_object")
Fixes: 7a81575b ("netfilter: nf_tables: add userdata attributes to nft_table")
Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>