tools headers UAPI: Sync linux/in.h copy with the kernel sources

To pick the changes in:

  db243b79 ("net/ipv4/ipv6: Replace one-element arraya with flexible-array members")
  2d3e5caf ("net/ipv4: Replace one-element array with flexible-array member")

That don't result in any change in tooling, the structs changed remains
with the same layout.

This addresses this build warning:

  Warning: Kernel ABI header at 'tools/include/uapi/linux/in.h' differs from latest version at 'include/uapi/linux/in.h'
  diff -u tools/include/uapi/linux/in.h include/uapi/linux/in.h

Cc: David S. Miller <davem@davemloft.net>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/include/uapi/linux/in.h

@@ -188,11 +188,22 @@ struct ip_mreq_source {
 };
 
 struct ip_msfilter {
-	__be32		imsf_multiaddr;
-	__be32		imsf_interface;
-	__u32		imsf_fmode;
-	__u32		imsf_numsrc;
-	__be32		imsf_slist[1];
+	union {
+		struct {
+			__be32		imsf_multiaddr_aux;
+			__be32		imsf_interface_aux;
+			__u32		imsf_fmode_aux;
+			__u32		imsf_numsrc_aux;
+			__be32		imsf_slist[1];
+		};
+		struct {
+			__be32		imsf_multiaddr;
+			__be32		imsf_interface;
+			__u32		imsf_fmode;
+			__u32		imsf_numsrc;
+			__be32		imsf_slist_flex[];
+		};
+	};
 };
 
 #define IP_MSFILTER_SIZE(numsrc) \
@@ -211,11 +222,22 @@ struct group_source_req {
 };
 
 struct group_filter {
-	__u32				 gf_interface;	/* interface index */
-	struct __kernel_sockaddr_storage gf_group;	/* multicast address */
-	__u32				 gf_fmode;	/* filter mode */
-	__u32				 gf_numsrc;	/* number of sources */
-	struct __kernel_sockaddr_storage gf_slist[1];	/* interface index */
+	union {
+		struct {
+			__u32				 gf_interface_aux; /* interface index */
+			struct __kernel_sockaddr_storage gf_group_aux;	   /* multicast address */
+			__u32				 gf_fmode_aux;	   /* filter mode */
+			__u32				 gf_numsrc_aux;	   /* number of sources */
+			struct __kernel_sockaddr_storage gf_slist[1];	   /* interface index */
+		};
+		struct {
+			__u32				 gf_interface;	  /* interface index */
+			struct __kernel_sockaddr_storage gf_group;	  /* multicast address */
+			__u32				 gf_fmode;	  /* filter mode */
+			__u32				 gf_numsrc;	  /* number of sources */
+			struct __kernel_sockaddr_storage gf_slist_flex[]; /* interface index */
+		};
+	};
 };
 
 #define GROUP_FILTER_SIZE(numsrc) \