We should check whether rtnetlink link operations
are defined before calling get_slave_size().

Without this, the following oops can occur when
adding a tap device to OVS.

[   87.839553] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a8
[   87.839595] IP: [<ffffffff813d47c0>] if_nlmsg_size+0xf0/0x220
[...]
[   87.840651] Call Trace:
[   87.840664]  [<ffffffff813d694b>] ? rtmsg_ifinfo+0x2b/0x100
[   87.840688]  [<ffffffff813c8340>] ? __netdev_adjacent_dev_insert+0x150/0x1a0
[   87.840718]  [<ffffffff813d6a50>] ? rtnetlink_event+0x30/0x40
[   87.840742]  [<ffffffff814b4144>] ? notifier_call_chain+0x44/0x70
[   87.840768]  [<ffffffff813c8946>] ? __netdev_upper_dev_link+0x3c6/0x3f0
[   87.840798]  [<ffffffffa0678d6c>] ? netdev_create+0xcc/0x160 [openvswitch]
[   87.840828]  [<ffffffffa06781ea>] ? ovs_vport_add+0x4a/0xd0 [openvswitch]
[   87.840857]  [<ffffffffa0670139>] ? new_vport+0x9/0x50 [openvswitch]
[   87.840884]  [<ffffffffa067279e>] ? ovs_vport_cmd_new+0x11e/0x210 [openvswitch]
[   87.840915]  [<ffffffff813f3efa>] ? genl_family_rcv_msg+0x19a/0x360
[   87.840941]  [<ffffffff813f40c0>] ? genl_family_rcv_msg+0x360/0x360
[   87.840967]  [<ffffffff813f4139>] ? genl_rcv_msg+0x79/0xc0
[   87.840991]  [<ffffffff813b6cf9>] ? __kmalloc_reserve.isra.25+0x29/0x80
[   87.841018]  [<ffffffff813f2389>] ? netlink_rcv_skb+0xa9/0xc0
[   87.841042]  [<ffffffff813f27cf>] ? genl_rcv+0x1f/0x30
[   87.841064]  [<ffffffff813f1988>] ? netlink_unicast+0xe8/0x1e0
[   87.841088]  [<ffffffff813f1d9a>] ? netlink_sendmsg+0x31a/0x750
[   87.841113]  [<ffffffff813aee96>] ? sock_sendmsg+0x86/0xc0
[   87.841136]  [<ffffffff813c960d>] ? __netdev_update_features+0x4d/0x200
[   87.841163]  [<ffffffff813ca94e>] ? ethtool_get_value+0x2e/0x50
[   87.841188]  [<ffffffff813af269>] ? ___sys_sendmsg+0x359/0x370
[   87.841212]  [<ffffffff813da686>] ? dev_ioctl+0x1a6/0x5c0
[   87.841236]  [<ffffffff8109c210>] ? autoremove_wake_function+0x30/0x30
[   87.841264]  [<ffffffff813ac59d>] ? sock_do_ioctl+0x3d/0x50
[   87.841288]  [<ffffffff813aca68>] ? sock_ioctl+0x1e8/0x2c0
[   87.841312]  [<ffffffff811934bf>] ? do_vfs_ioctl+0x2cf/0x4b0
[   87.841335]  [<ffffffff813afeb9>] ? __sys_sendmsg+0x39/0x70
[   87.841362]  [<ffffffff814b86f9>] ? system_call_fastpath+0x16/0x1b
[   87.841386] Code: c0 74 10 48 89 ef ff d0 83 c0 07 83 e0 fc 48 98 49 01 c7 48 89 ef e8 d0 d6 fe ff 48 85 c0 0f 84 df 00 00 00 48 8b 90 08 07 00 00 <48> 8b 8a a8 00 00 00 31 d2 48 85 c9 74 0c 48 89 ee 48 89 c7 ff
[   87.841529] RIP  [<ffffffff813d47c0>] if_nlmsg_size+0xf0/0x220
[   87.841555]  RSP <ffff880221aa5950>
[   87.841569] CR2: 00000000000000a8
[   87.851442] ---[ end trace e42ab217691b4fc2 ]---

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>