sunrpc: Hash uids by first computing their value in the initial userns

In svcauth_unix introduce a helper unix_gid_hash as otherwise the
expresion to generate the hash value is just too long.

Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
index 4cd0ecf..392adc4 100644
--- a/net/sunrpc/auth.c
+++ b/net/sunrpc/auth.c
@@ -412,7 +412,7 @@
 			*entry, *new;
 	unsigned int nr;
 
-	nr = hash_long(acred->uid, cache->hashbits);
+	nr = hash_long(from_kuid(&init_user_ns, acred->uid), cache->hashbits);
 
 	rcu_read_lock();
 	hlist_for_each_entry_rcu(entry, pos, &cache->hashtable[nr], cr_hash) {