[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/net/ipv4/inet_diag.c b/net/ipv4/inet_diag.c
index 0148f0e..dbeacd8 100644
--- a/net/ipv4/inet_diag.c
+++ b/net/ipv4/inet_diag.c
@@ -893,7 +893,7 @@
 		goto out;
 
 	idiagnl = netlink_kernel_create(NETLINK_INET_DIAG, 0, inet_diag_rcv,
-					THIS_MODULE);
+					NULL, THIS_MODULE);
 	if (idiagnl == NULL)
 		goto out_free_table;
 	err = 0;