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:
Cc: Josh Poimboeuf <jpoimboe@redhat.com> Signed-off-by: Jeremy Cline <jcline@redhat.com> Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net>