netfilter: nf_tables: use nla_memdup to copy udata

When userdata support was added to tables and objects, user data coming
from user space was allocated and copied using kzalloc + nla_memcpy.

Use nla_memdup to copy userdata of tables and objects.

Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>