RDS: fix rds-ping spinlock recursion

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 <dan.carpenter@oracle.com>
CC: Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>
CC: David S. Miller <davem@davemloft.net>
CC: James Morris <james.l.morris@oracle.com>
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 5175a5e76bbdf20a614fb47ce7a38f0f39e70226)

Signed-off-by: Jerry Snitselaar <jerry.snitselaar@oracle.com>
Conflicts:
	net/rds/send.c

Orabug: 16223050

Signed-off-by: Jerry Snitselaar <dev@snitselaar.org>
(cherry picked from commit 3badb20f7c232c2f72758453d01cb890ab686def)

Signed-off-by: Jerry Snitselaar <jerry.snitselaar@oracle.com>
(cherry picked from commit bd05c6c016b911bb7d9a16f2998389b4219bb2cf)