NLM: nlm_privileged_requester() doesn't recognize mapped loopback address

Commit b85e4676 added the nlm_privileged_requester() helper to check
whether an RPC request was sent from a local privileged caller.  It
recognizes IPv4 privileged callers (from "127.0.0.1"), and IPv6
privileged callers (from "::1").

However, IPV6_ADDR_LOOPBACK is not set for the mapped IPv4 loopback
address (::ffff:7f00:0001), so the test breaks when the kernel's RPC
service is IPv6-enabled but user space is calling via the IPv4
loopback address.  This is actually the most common case for IPv6-
enabled RPC services on Linux.

Rewrite the IPv6 check to handle the mapped IPv4 loopback address as
well as a normal IPv6 loopback address.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

include/linux/lockd/lockd.h

@@ -299,8 +299,14 @@ static inline int __nlm_privileged_request4(const struct sockaddr *sap)
 static inline int __nlm_privileged_request6(const struct sockaddr *sap)
 {
 	const struct sockaddr_in6 *sin6 = (struct sockaddr_in6 *)sap;
-	return (ipv6_addr_type(&sin6->sin6_addr) & IPV6_ADDR_LOOPBACK) &&
-			(ntohs(sin6->sin6_port) < 1024);
+
+	if (ntohs(sin6->sin6_port) > 1023)
+		return 0;
+
+	if (ipv6_addr_type(&sin6->sin6_addr) & IPV6_ADDR_MAPPED)
+		return ipv4_is_loopback(sin6->sin6_addr.s6_addr32[3]);
+
+	return ipv6_addr_type(&sin6->sin6_addr) & IPV6_ADDR_LOOPBACK;
 }
 #else	/* defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) */
 static inline int __nlm_privileged_request6(const struct sockaddr *sap)