returned, and its RPC_BC_PREALLOC_IN_USE bit is set in
rpc_xprt->bc_flags.  The structure is removed from the the list
since it is now in use, and it will be later added back when its
user is done with it.

After the RPC callback replies, the rpc_rqst structure is returned
by invoking xprt_free_bc_request().  This clears the
RPC_BC_PREALLOC_IN_USE bit and adds it back to the list, allowing it
to be reused by a subsequent RPC callback request.

To be consistent with the reception of RPC messages, the backchannel requests
should be placed into the 'struct rpc_rqst' rq_rcv_buf, which is then in turn
copied to the 'struct rpc_rqst' rq_private_buf.

[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>
[Update copyright notice and explain page allocation]
Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
+…³8˜