netfilter: bridge: port sysctls to use brnf_net

netfilter: bridge: port sysctls to use brnf_net

BugLink: https://bugs.launchpad.net/bugs/1836910
This ports the sysctls to use struct brnf_net.

With this patch we make it possible to namespace the br_netfilter module in
the following patch.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
(cherry picked from commit ff6d090d0db41425aef0cfe5dc58bb3cc12514a2)
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>