dst: rcu check refinement

__sk_dst_get() might be called from softirq, with socket lock held.

[  159.026180] include/net/sock.h:1200 invoked rcu_dereference_check()
without protection!
[  159.026261] 
[  159.026261] other info that might help us debug this:
[  159.026263] 
[  159.026425] 
[  159.026426] rcu_scheduler_active = 1, debug_locks = 0
[  159.026552] 2 locks held by swapper/0:
[  159.026609]  #0:  (&icsk->icsk_retransmit_timer){+.-...}, at:
[<ffffffff8104fc15>] run_timer_softirq+0x105/0x350
[  159.026839]  #1:  (slock-AF_INET){+.-...}, at: [<ffffffff81392b8f>]
tcp_write_timer+0x2f/0x1e0
[  159.027063] 
[  159.027064] stack backtrace:
[  159.027172] Pid: 0, comm: swapper Not tainted
2.6.34-rc5-03707-gde498c8-dirty #36
[  159.027252] Call Trace:
[  159.027306]  <IRQ>  [<ffffffff810718ef>] lockdep_rcu_dereference
+0xaf/0xc0
[  159.027411]  [<ffffffff8138e4f7>] tcp_current_mss+0xa7/0xb0
[  159.027537]  [<ffffffff8138fa49>] tcp_write_wakeup+0x89/0x190
[  159.027600]  [<ffffffff81391936>] tcp_send_probe0+0x16/0x100
[  159.027726]  [<ffffffff81392cd9>] tcp_write_timer+0x179/0x1e0
[  159.027790]  [<ffffffff8104fca1>] run_timer_softirq+0x191/0x350
[  159.027980]  [<ffffffff810477ed>] __do_softirq+0xcd/0x200


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