Make linux(4) support SO_PROTOCOL. Running Python test suite

with python3.8 from Focal triggers those.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25491

sys/compat/linux/linux_socket.c

@@ -236,6 +236,8 @@ linux_to_bsd_so_sockopt(int opt)
 		return (SO_TIMESTAMP);
 	case LINUX_SO_ACCEPTCONN:
 		return (SO_ACCEPTCONN);
+	case LINUX_SO_PROTOCOL:
+		return (SO_PROTOCOL);
 	}
 	return (-1);
 }

sys/compat/linux/linux_socket.h

@@ -197,6 +197,7 @@ int linux_accept(struct thread *td, struct linux_accept_args *args);
 #define	LINUX_SO_ACCEPTCONN	30
 #define	LINUX_SO_SNDBUFFORCE	32
 #define	LINUX_SO_RCVBUFFORCE	33
+#define	LINUX_SO_PROTOCOL	38
 
 /* Socket options */
 #define	LINUX_IP_TOS		1