netfilter: nf_tables: add support to destroy operation

Introduce NFT_MSG_DESTROY* message type. The destroy operation performs a
delete operation but ignoring the ENOENT errors.

This is useful for the transaction semantics, where failing to delete an
object which does not exist results in aborting the transaction.

This new command allows the transaction to proceed in case the object
does not exist.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>