netfilter: nf_tables: extend NFT_MSG_DELTABLE to support flushing the ruleset

This patch extend the NFT_MSG_DELTABLE call to support flushing the entire
ruleset.

The options now are:
 * No family speficied, no table specified: flush all the ruleset.
 * Family specified, no table specified: flush all tables in the AF.
 * Family specified, table specified: flush the given table.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>