bridge: provide a cow_metrics method for fake_ops

Like in commit 0972ddb2 (provide cow_metrics() methods to blackhole
dst_ops), we must provide a cow_metrics for bridges fake_dst_ops as
well.

This fixes a regression coming from commits 62fa8a84 (net: Implement
read-only protection and COW'ing of metrics.) and 33eb9873 (bridge:
initialize fake_rtable metrics)

ip link set mybridge mtu 1234
-->
[  136.546243] Pid: 8415, comm: ip Tainted: P 
2.6.39.1-00006-g40545b7 #103 ASUSTeK Computer Inc.         V1Sn 
        /V1Sn
[  136.546256] EIP: 0060:[<00000000>] EFLAGS: 00010202 CPU: 0
[  136.546268] EIP is at 0x0
[  136.546273] EAX: f14a389c EBX: 000005d4 ECX: f80d32c0 EDX: f80d1da1
[  136.546279] ESI: f14a3000 EDI: f255bf10 EBP: f15c3b54 ESP: f15c3b48
[  136.546285]  DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
[  136.546293] Process ip (pid: 8415, ti=f15c2000 task=f4741f80 
task.ti=f15c2000)
[  136.546297] Stack:
[  136.546301]  f80c658f f14a3000 ffffffed f15c3b64 c12cb9c8 f80d1b80 
ffffffa1 f15c3bbc
[  136.546315]  c12da347 c12d9c7d 00000000 f7670b00 00000000 f80d1b80 
ffffffa6 f15c3be4
[  136.546329]  00000004 f14a3000 f255bf20 00000008 f15c3bbc c11d6cae 
00000000 00000000
[  136.546343] Call Trace:
[  136.546359]  [<f80c658f>] ? br_change_mtu+0x5f/0x80 [bridge]
[  136.546372]  [<c12cb9c8>] dev_set_mtu+0x38/0x80
[  136.546381]  [<c12da347>] do_setlink+0x1a7/0x860
[  136.546390]  [<c12d9c7d>] ? rtnl_fill_ifinfo+0x9bd/0xc70
[  136.546400]  [<c11d6cae>] ? nla_parse+0x6e/0xb0
[  136.546409]  [<c12db931>] rtnl_newlink+0x361/0x510
[  136.546420]  [<c1023240>] ? vmalloc_sync_all+0x100/0x100
[  136.546429]  [<c1362762>] ? error_code+0x5a/0x60
[  136.546438]  [<c12db5d0>] ? rtnl_configure_link+0x80/0x80
[  136.546446]  [<c12db27a>] rtnetlink_rcv_msg+0xfa/0x210
[  136.546454]  [<c12db180>] ? __rtnl_unlock+0x20/0x20
[  136.546463]  [<c12ee0fe>] netlink_rcv_skb+0x8e/0xb0
[  136.546471]  [<c12daf1c>] rtnetlink_rcv+0x1c/0x30
[  136.546479]  [<c12edafa>] netlink_unicast+0x23a/0x280
[  136.546487]  [<c12ede6b>] netlink_sendmsg+0x26b/0x2f0
[  136.546497]  [<c12bb828>] sock_sendmsg+0xc8/0x100
[  136.546508]  [<c10adf61>] ? __alloc_pages_nodemask+0xe1/0x750
[  136.546517]  [<c11d0602>] ? _copy_from_user+0x42/0x60
[  136.546525]  [<c12c5e4c>] ? verify_iovec+0x4c/0xc0
[  136.546534]  [<c12bd805>] sys_sendmsg+0x1c5/0x200
[  136.546542]  [<c10c2150>] ? __do_fault+0x310/0x410
[  136.546549]  [<c10c2c46>] ? do_wp_page+0x1d6/0x6b0
[  136.546557]  [<c10c47d1>] ? handle_pte_fault+0xe1/0x720
[  136.546565]  [<c12bd1af>] ? sys_getsockname+0x7f/0x90
[  136.546574]  [<c10c4ec1>] ? handle_mm_fault+0xb1/0x180
[  136.546582]  [<c1023240>] ? vmalloc_sync_all+0x100/0x100
[  136.546589]  [<c10233b3>] ? do_page_fault+0x173/0x3d0
[  136.546596]  [<c12bd87b>] ? sys_recvmsg+0x3b/0x60
[  136.546605]  [<c12bdd83>] sys_socketcall+0x293/0x2d0
[  136.546614]  [<c13629d0>] sysenter_do_call+0x12/0x26
[  136.546619] Code:  Bad EIP value.
[  136.546627] EIP: [<00000000>] 0x0 SS:ESP 0068:f15c3b48
[  136.546645] CR2: 0000000000000000
[  136.546652] ---[ end trace 6909b560e78934fa ]---

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