bridge: move br_net_exit() to br.c

And it can become static.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/bridge/br.c
Simple merge

diff --cc net/bridge/br_if.c
Simple merge

diff --cc net/bridge/br_private.h
Simple merge