[PATCH] RPC: make sure to get the same local port number when reconnecting

 Implement a best practice: if the remote end drops our connection, try to
 reconnect using the same port number.  This is important because the NFS
 server's Duplicate Reply Cache often hashes on the source port number.
 If the client reuses the port number when it reconnects, the server's DRC
 will be more effective.

 Based on suggestions by Mike Eisler, Olaf Kirch, and Alexey Kuznetsky.

 Test-plan:
 Destructive testing.

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