sunrpc: Fix reserved port range calculation

The range calculation for choosing the random reserved port will panic
with divide-by-zero when min_resvport == max_resvport, a range of one
port, not zero.

Fix the reserved port range calculation by adding one to the difference.
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprtsock.c

@@ -1714,7 +1714,7 @@ static void xs_udp_timer(struct rpc_xprt *xprt, struct rpc_task *task)
 
 static unsigned short xs_get_random_port(void)
 {
-	unsigned short range = xprt_max_resvport - xprt_min_resvport;
+	unsigned short range = xprt_max_resvport - xprt_min_resvport + 1;
 	unsigned short rand = (unsigned short) prandom_u32() % range;
 	return rand + xprt_min_resvport;
 }