NFSv4.1: Don't rebind to the same source port when reconnecting to the server

NFSv2, v3 and NFSv4 servers often have duplicate replay caches that look
at the source port when deciding whether or not an RPC call is a replay
of a previous call. This requires clients to perform strange TCP gymnastics
in order to ensure that when they reconnect to the server, they bind
to the same source port.

NFSv4.1 and NFSv4.2 have sessions that provide proper replay semantics,
that do not look at the source port of the connection. This patch therefore
ensures they can ignore the rebind requirement.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

include/linux/sunrpc/xprt.h

@@ -207,7 +207,8 @@ struct rpc_xprt {
 	unsigned int		min_reqs;	/* min number of slots */
 	unsigned int		num_reqs;	/* total slots */
 	unsigned long		state;		/* transport state */
-	unsigned char		resvport   : 1; /* use a reserved port */
+	unsigned char		resvport   : 1,	/* use a reserved port */
+				reuseport  : 1; /* reuse port on reconnect */
 	atomic_t		swapper;	/* we're swapping over this
 						   transport */
 	unsigned int		bind_index;	/* bind function index */