netlink: check for null extack in cookie helpers

Unlike NL_SET_ERR_* macros, nl_set_extack_cookie_u64() and
nl_set_extack_cookie_u32() helpers do not check extack argument for null
and neither do their callers, as syzbot recently discovered for
ethnl_parse_header().

Instead of fixing the callers and leaving the trap in place, add check of
null extack to both helpers to make them consistent with NL_SET_ERR_*
macros.

v2: drop incorrect second Fixes tag

Fixes: 2363d73a ("ethtool: reject unrecognized request flags")
Reported-by: syzbot+258a9089477493cea67b@syzkaller.appspotmail.com
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -115,6 +115,8 @@ static inline void nl_set_extack_cookie_u64(struct netlink_ext_ack *extack,
 {
 	u64 __cookie = cookie;
 
+	if (!extack)
+		return;
 	memcpy(extack->cookie, &__cookie, sizeof(__cookie));
 	extack->cookie_len = sizeof(__cookie);
 }
@@ -124,6 +126,8 @@ static inline void nl_set_extack_cookie_u32(struct netlink_ext_ack *extack,
 {
 	u32 __cookie = cookie;
 
+	if (!extack)
+		return;
 	memcpy(extack->cookie, &__cookie, sizeof(__cookie));
 	extack->cookie_len = sizeof(__cookie);
 }