sock_diag: fix autoloading of the raw_diag module

IPPROTO_RAW isn't registred as an inet protocol, so
inet_protos[protocol] is always NULL for it.

Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Xin Long <lucien.xin@gmail.com>
Fixes: bf2ae2e4

 ("sock_diag: request _diag module only when the family or proto has been registered")
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>