IPv6: convert mc_lock to spinlock

Only used for writing, so convert to spinlock

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/if_inet6.h b/include/net/if_inet6.h
index e9d69d1..545d8b0 100644
--- a/include/net/if_inet6.h
+++ b/include/net/if_inet6.h
@@ -157,7 +157,7 @@
 
 	struct ifmcaddr6	*mc_list;
 	struct ifmcaddr6	*mc_tomb;
-	rwlock_t		mc_lock;
+	spinlock_t		mc_lock;
 	unsigned char		mc_qrv;
 	unsigned char		mc_gq_running;
 	unsigned char		mc_ifc_count;