RDS: fix rds-ping spinlock recursion

This is the revised patch for fixing rds-ping spinlock recursion
according to Venkat's suggestions.

RDS ping/pong over TCP feature has been broken for years(2.6.39 to
3.6.0) since we have to set TCP cork and call kernel_sendmsg() between
ping/pong which both need to lock "struct sock *sk". However, this
lock has already been hold before rds_tcp_data_ready() callback is
triggerred. As a result, we always facing spinlock resursion which
would resulting in system panic.

Given that RDS ping is only used to test the connectivity and not for
serious performance measurements, we can queue the pong transmit to
rds_wq as a delayed response.

Reported-by: Dan Carpenter <[e-mail]>
CC: Venkat Venkatsubra <[e-mail]>
CC: David S. Miller <[e-mail]>
CC: James Morris <[e-mail]>
Signed-off-by: Jie Liu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/rds/send.c

···
 	rds_stats_inc(s_send_pong);
 
 	if (!test_bit(RDS_LL_SEND_FULL, &conn->c_flags))
-		rds_send_xmit(conn);
+		queue_delayed_work(rds_wq, &conn->c_send_w, 0);
 
 	rds_message_put(rm);
 	return 0;