[PATCH] RPC: rationalize set_buffer_size

 In fact, ->set_buffer_size should be completely functionless for non-UDP.

 Test-plan:
 Check socket buffer size on UDP sockets over time.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>