when several RPC requests want to reconnect a TCP transport socket at
once, xprt_lock_write serializes the tasks to prevent multiple socket
connects.  however, TCP connects are always done by a RPC child task that
has no request slot.  xprt_lock_write can oops if there is no request slot
allocated to the invoking RPC task.  reviewed and accepted by Trond.

the xprt_lock_write changes are not yet in 2.4, so this patch does not
apply to 2.4.