netfilter: nf_tables: split chain policy validation from actually setting it

Currently nf_tables_newchain() atomicity is broken because of having
validation of some netlink attributes performed after changing attributes
of the chain. The chain policy is (currently) fine, but split it up as
preparation for the following fixes and to avoid future mistakes.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>