[PATCH] RPC: add generic interface for adjusting the congestion window

 A new interface that allows transports to adjust their congestion window
 using the Van Jacobson implementation in xprt.c is provided.

 Test-plan:
 Use WAN simulation to cause sporadic bursty packet loss.  Look for
 significant regression in performance or client stability.

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