netfilter: nf_tables: do not send complete notification of deletions

In most cases, table, name and handle is sufficient for userspace to
identify an object that has been deleted. Skipping unneeded fields in
the netlink attributes in the message saves bandwidth (ie. less chances
of hitting ENOBUFS).

Rules are an exception: the existing userspace monitor code relies on
the rule definition. This exception can be removed by implementing a
rule cache in userspace, this is already supported by the tracing
infrastructure.

Regarding flowtables, incremental deletion of devices is possible.
Skipping a full notification allows userspace to differentiate between
flowtable removal and incremental removal of devices.

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