netfilter: bridge: Expose nf_tables bridge hook priorities through uapi

Netfilter exposes standard hook priorities in case of ipv4, ipv6 and
arp but not in case of bridge.

This patch exposes the hook priority values of the bridge family (which are
different from the formerly mentioned) via uapi so that they can be used by
user-space applications just like the others.

Signed-off-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>