tcp: fix inet_twsk_deschedule()

Eric W. Biederman reported a lockdep splat in inet_twsk_deschedule()

This is caused by inet_twsk_purge(), run from process context,
and commit 575f4cd5 (net: Use rcu lookups in inet_twsk_purge.)
removed the BH disabling that was necessary.

Add the BH disabling but fine grained, right before calling
inet_twsk_deschedule(), instead of whole function.

With help from Linus Torvalds and Eric W. Biederman
Reported-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
CC: Daniel Lezcano <daniel.lezcano@free.fr>
CC: Pavel Emelyanov <xemul@openvz.org>
CC: Arnaldo Carvalho de Melo <acme@redhat.com>
CC: stable <stable@kernel.org> (# 2.6.33+)
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/inet_timewait_sock.c

@@ -505,7 +505,9 @@ void inet_twsk_purge(struct inet_hashinfo *hashinfo,
 			}
 
 			rcu_read_unlock();
+			local_bh_disable();
 			inet_twsk_deschedule(tw, twdr);
+			local_bh_enable();
 			inet_twsk_put(tw);
 			goto restart_rcu;
 		}