netfilter: nf_tables: add position handle in event notification

netfilter: nf_tables: add position handle in event notification

Add position handle to allow to identify the rule location from netlink
events. Otherwise, userspace cannot incrementally update a userspace
cache through monitoring events.

Skip handle dump if the rule has been either inserted (at the beginning
of the ruleset) or appended (at the end of the ruleset), the
NLM_F_APPEND netlink flag is sufficient in these two cases.

Handle NLM_F_REPLACE as NLM_F_APPEND since the rule replacement
expansion appends it after the specified rule handle.

Fixes: 4a4835e91e03 ("netfilter: add nftables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>