[NETLINK]: Switch cb_lock spinlock to mutex and allow to override it

Switch cb_lock to mutex and allow netlink kernel users to override it
with a subsystem specific mutex for consistent locking in dump callbacks.
All netlink_dump_start users have been audited not to rely on any
side-effects of the previously used spinlock.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/connector/connector.c b/drivers/connector/connector.c
index 7f9c4fb..a7b9e9b 100644
--- a/drivers/connector/connector.c
+++ b/drivers/connector/connector.c
@@ -448,7 +448,7 @@
 
 	dev->nls = netlink_kernel_create(NETLINK_CONNECTOR,
 					 CN_NETLINK_USERS + 0xf,
-					 dev->input, THIS_MODULE);
+					 dev->input, NULL, THIS_MODULE);
 	if (!dev->nls)
 		return -EIO;