ipv6: fix redefinition of in6_pktinfo and ip6_mtuinfo

Both netinet/in.h and linux/ipv6.h define these two structs,
if we include both of them, we got:

	/usr/include/linux/ipv6.h:19:8: error: redefinition of ‘struct in6_pktinfo’
	 struct in6_pktinfo {
		^
	In file included from /usr/include/arpa/inet.h:22:0,
			 from txtimestamp.c:33:
	/usr/include/netinet/in.h:524:8: note: originally defined here
	 struct in6_pktinfo
		^
	In file included from txtimestamp.c:40:0:
	/usr/include/linux/ipv6.h:24:8: error: redefinition of ‘struct ip6_mtuinfo’
	 struct ip6_mtuinfo {
		^
	In file included from /usr/include/arpa/inet.h:22:0,
			 from txtimestamp.c:33:
	/usr/include/netinet/in.h:531:8: note: originally defined here
	 struct ip6_mtuinfo
		^
So similarly to what we did for in6_addr, we need to sync with
libc header on their definitions.
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/ipv6.h

@@ -15,16 +15,19 @@
  *	*under construction*
  */
 
-
+#if __UAPI_DEF_IN6_PKTINFO
 struct in6_pktinfo {
 	struct in6_addr	ipi6_addr;
 	int		ipi6_ifindex;
 };
+#endif
 
+#if __UAPI_DEF_IP6_MTUINFO
 struct ip6_mtuinfo {
 	struct sockaddr_in6	ip6m_addr;
 	__u32			ip6m_mtu;
 };
+#endif
 
 struct in6_ifreq {
 	struct in6_addr	ifr6_addr;

include/uapi/linux/libc-compat.h

@@ -70,6 +70,8 @@
 #define __UAPI_DEF_IPV6_MREQ		0
 #define __UAPI_DEF_IPPROTO_V6		0
 #define __UAPI_DEF_IPV6_OPTIONS		0
+#define __UAPI_DEF_IN6_PKTINFO		0
+#define __UAPI_DEF_IP6_MTUINFO		0
 
 #else
 
@@ -84,6 +86,8 @@
 #define __UAPI_DEF_IPV6_MREQ		1
 #define __UAPI_DEF_IPPROTO_V6		1
 #define __UAPI_DEF_IPV6_OPTIONS		1
+#define __UAPI_DEF_IN6_PKTINFO		1
+#define __UAPI_DEF_IP6_MTUINFO		1
 
 #endif /* _NETINET_IN_H */
 
@@ -106,6 +110,8 @@
 #define __UAPI_DEF_IPV6_MREQ		1
 #define __UAPI_DEF_IPPROTO_V6		1
 #define __UAPI_DEF_IPV6_OPTIONS		1
+#define __UAPI_DEF_IN6_PKTINFO		1
+#define __UAPI_DEF_IP6_MTUINFO		1
 
 /* Definitions for xattr.h */
 #define __UAPI_DEF_XATTR		1