src: Allow reset single stateful object

src: Allow reset single stateful object

Currently the stateful objects can only be reseted in groups. With this
patch reseting a single object is allowed:

$ nft reset counter filter https-traffic
table ip filter {
	counter https-traffic {
		packets 8774 bytes 542668
	}
}

$ nft list counter filter https-traffic
table ip filter {
	counter https-traffic {
		packets 0 bytes 0
	}
}

Heavily based on work from Pablo Neira Ayuso <pablo@netfilter.org>.

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>