[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>

security/selinux/netlink.c

@@ -104,7 +104,7 @@ void selnl_notify_policyload(u32 seqno)
 
 static int __init selnl_init(void)
 {
-	selnl = netlink_kernel_create(NETLINK_SELINUX, SELNLGRP_MAX, NULL,
+	selnl = netlink_kernel_create(NETLINK_SELINUX, SELNLGRP_MAX, NULL, NULL,
 	                              THIS_MODULE);
 	if (selnl == NULL)
 		panic("SELinux:  Cannot create netlink socket.");