net: ipv6: fix oops in inet_putpeer()

Commit 97bab73f (inet: Hide route peer accesses behind helpers.) introduced
a bug in xfrm6_policy_destroy(). The xfrm_dst's _rt6i_peer member is not
initialized, causing a false positive result from inetpeer_ptr_is_peer(),
which in turn causes a NULL pointer dereference in inet_putpeer().

Pid: 314, comm: kworker/0:1 Not tainted 3.6.0-rc1+ #17 To Be Filled By O.E.M. To Be Filled By O.E.M./P4S800D-X
EIP: 0060:[<c03abf93>] EFLAGS: 00010246 CPU: 0
EIP is at inet_putpeer+0xe/0x16
EAX: 00000000 EBX: f3481700 ECX: 00000000 EDX: 000dd641
ESI: f3481700 EDI: c05e949c EBP: f551def4 ESP: f551def4
 DS: 007b ES: 007b FS: 0000 GS: 00e0 SS: 0068
CR0: 8005003b CR2: 00000070 CR3: 3243d000 CR4: 00000750
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff0ff0 DR7: 00000400
 f551df04 c0423de1 00000000 f3481700 f551df18 c038d5f7 f254b9f8 f551df28
 f34f85d8 f551df20 c03ef48d f551df3c c0396870 f30697e8 f24e1738 c05e98f4
 f5509540 c05cd2b4 f551df7c c0142d2b c043feb5 f5509540 00000000 c05cd2e8
 [<c0423de1>] xfrm6_dst_destroy+0x42/0xdb
 [<c038d5f7>] dst_destroy+0x1d/0xa4
 [<c03ef48d>] xfrm_bundle_flo_delete+0x2b/0x36
 [<c0396870>] flow_cache_gc_task+0x85/0x9f
 [<c0142d2b>] process_one_work+0x122/0x441
 [<c043feb5>] ? apic_timer_interrupt+0x31/0x38
 [<c03967eb>] ? flow_cache_new_hashrnd+0x2b/0x2b
 [<c0143e2d>] worker_thread+0x113/0x3cc

Fix by adding a init_dst() callback to struct xfrm_policy_afinfo to
properly initialize the dst's peer pointer.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>