Make linux(4) handle SO_REUSEPORT.

Reviewed by:	emaste
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25216

sys/compat/linux/linux_socket.c

@@ -223,6 +223,8 @@ linux_to_bsd_so_sockopt(int opt)
 		return (SO_OOBINLINE);
 	case LINUX_SO_LINGER:
 		return (SO_LINGER);
+	case LINUX_SO_REUSEPORT:
+		return (SO_REUSEPORT_LB);
 	case LINUX_SO_PEERCRED:
 		return (LOCAL_PEERCRED);
 	case LINUX_SO_RCVLOWAT:

sys/compat/linux/linux_socket.h

@@ -190,6 +190,7 @@ int linux_accept(struct thread *td, struct linux_accept_args *args);
 #define	LINUX_SO_NO_CHECK	11
 #define	LINUX_SO_PRIORITY	12
 #define	LINUX_SO_LINGER		13
+#define	LINUX_SO_REUSEPORT	15
 #ifndef LINUX_SO_PASSCRED	/* powerpc differs */
 #define	LINUX_SO_PASSCRED	16
 #define	LINUX_SO_PEERCRED	17