netfilter: x_tables: introduce and use xt_copy_counters_from_user

commit d7591f0c41ce3e67600a982bab6989ef0f07b3ce upstream.

The three variants use same copy&pasted code, condense this into a
helper and use that.

Make sure info.name is 0-terminated.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>