sock_writeable determines whether there is space in a socket's output
buffer.  socket write_space callbacks use it to determine whether to wake
up those that are waiting for more output buffer space.

however, sock_writeable is not appropriate for TCP sockets.  because the
RPC layer's write_space callback uses it for TCP sockets, the RPC layer
hammers on sock_sendmsg with dozens of write requests that are only a few
hundred bytes long when it is trying to send a large write RPC request.
this patch adds logic to the RPC layer's write_space callback that
properly handles TCP sockets.

patch reviewed by Trond and Alexey.