netlink: Fix spectre v1 gadget in netlink_create()

'protocol' is a user-controlled value, so sanitize it after the bounds
check to avoid using it for speculative out-of-bounds access to arrays
indexed by it.

This addresses the following accesses detected with the help of smatch:

* net/netlink/af_netlink.c:654 __netlink_create() warn: potential
spectre issue 'nlk_cb_mutex_keys' [w]

* net/netlink/af_netlink.c:654 __netlink_create() warn: potential
spectre issue 'nlk_cb_mutex_key_strings' [w]

* net/netlink/af_netlink.c:685 netlink_create() warn: potential spectre
issue 'nl_table' [w] (local cap)

Cc: Josh Poimboeuf <[e-mail]>
Signed-off-by: Jeremy Cline <[e-mail]>
Reviewed-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/netlink/af_netlink.c

···
 #include <linux/hash.h>
 #include <linux/genetlink.h>
 #include <linux/net_namespace.h>
+#include <linux/nospec.h>
 
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
···
 
 	if (protocol < 0 || protocol >= MAX_LINKS)
 		return -EPROTONOSUPPORT;
+	protocol = array_index_nospec(protocol, MAX_LINKS);
 
 	netlink_lock_table();
 #ifdef CONFIG_MODULES