netfilter: nf_tables: add NFT_LOGLEVEL_* enumeration and use it

This is internal, not exposed through uapi, and although it maps with
userspace LOG_*, with the introduction of LOGLEVEL_AUDIT we are
incurring in namespace pollution.

This patch adds the NFT_LOGLEVEL_ enumeration and use it from nft_log.

Fixes: 1a893b44

 ("netfilter: nf_tables: Add audit support to log statement")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>