sunrpc: fix write space race causing stalls

Write space becoming available may race with putting the task to sleep
in xprt_wait_for_buffer_space().  The existing mechanism to avoid the
race does not work.

This (edited) partial trace illustrates the problem:

   [1] rpc_task_run_action: task:43546@5 ... action=call_transmit
   [2] xs_write_space <-xs_tcp_write_space
   [3] xprt_write_space <-xs_write_space
   [4] rpc_task_sleep: task:43546@5 ...
   [5] xs_write_space <-xs_tcp_write_space

[1] Task 43546 runs but is out of write space.

[2] Space becomes available, xs_write_space() clears the
    SOCKWQ_ASYNC_NOSPACE bit.

[3] xprt_write_space() attemts to wake xprt->snd_task (== 43546), but
    this has not yet been queued and the wake up is lost.

[4] xs_nospace() is called which calls xprt_wait_for_buffer_space()
    which queues task 43546.

[5] The call to sk->sk_write_space() at the end of xs_nospace() (which
    is supposed to handle the above race) does not call
    xprt_write_space() as the SOCKWQ_ASYNC_NOSPACE bit is clear and
    thus the task is not woken.

Fix the race by resetting the SOCKWQ_ASYNC_NOSPACE bit in xs_nospace()
so the second call to sk->sk_write_space() calls xprt_write_space().

Suggested-by: Trond Myklebust <trondmy@primarydata.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
cc: stable@vger.kernel.org # 4.4
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>