netfilter: nf_tables: simplify nf_tables_*_notify

Now that all these function are called from the commit path, we can
pass the context structure to reduce the amount of parameters in all
of the nf_tables_*_notify functions. This patch also removes unneeded
branches to check for skb, nlh and net that should be always set in
the context structure.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>