Handles RPC replies and backchannel callbacks.  Traditionally the NFS
client has expected only RPC replies on its open connections.  With
NFSv4.1, callbacks can arrive over an existing open connection.

This patch refactors the old xs_tcp_read_request() into an RPC reply handler:
xs_tcp_read_reply(), a new backchannel callback handler: xs_tcp_read_callback(),
and a common routine to read the data off the transport: xs_tcp_read_common().
The new xs_tcp_read_callback() queues callback requests onto a queue where
the callback service (a separate thread) is listening for the processing.

This patch incorporates work and suggestions from Rahul Iyer (iyer@netapp.com)
and Benny Halevy (bhalevy@panasas.com).

xs_tcp_read_callback() drops the connection when the number of expected
callbacks is exceeded.  Use xprt_force_disconnect(), ensuring tasks on
the pending queue are awaken on disconnect.

[nfs41: Keep track of RPC call/reply direction with a flag]
[nfs41: Preallocate rpc_rqst receive buffer for handling callbacks]
Signed-off-by: Ricardo Labiaga <ricardo.labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: sunrpc: xs_tcp_read_callback() should use xprt_force_disconnect()]
Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[Moves embedded #ifdefs into #ifdef function blocks]
Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>