bridge: initialize fake_rtable metrics

bridge netfilter code uses a fake_rtable, and we must init its _metric
field or risk NULL dereference later.

Ref: https://bugzilla.kernel.org/show_bug.cgi?id=35672



Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>