ipv6: fib6_new_sernum() optimization

Adopt atomic_try_cmpxchg() which is slightly more efficient.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: David Ahern <dsahern@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -91,13 +91,12 @@ static void fib6_walker_unlink(struct net *net, struct fib6_walker *w)
 
 static int fib6_new_sernum(struct net *net)
 {
-	int new, old;
+	int new, old = atomic_read(&net->ipv6.fib6_sernum);
 
 	do {
-		old = atomic_read(&net->ipv6.fib6_sernum);
 		new = old < INT_MAX ? old + 1 : 1;
-	} while (atomic_cmpxchg(&net->ipv6.fib6_sernum,
-				old, new) != old);
+	} while (!atomic_try_cmpxchg(&net->ipv6.fib6_sernum, &old, new));
+
 	return new;
 }