userns: use union in {g,u}idmap struct

- Add a struct containing two pointer to extents and wrap both the static extent
  array and the struct into a union. This is done in preparation for bumping the
  {g,u}idmap limits for user namespaces.
- Add brackets around anonymous union when using designated initializers to
  initialize members in order to please gcc <= 4.4.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

kernel/user.c

@@ -26,26 +26,32 @@
 struct user_namespace init_user_ns = {
 	.uid_map = {
 		.nr_extents = 1,
-		.extent[0] = {
-			.first = 0,
-			.lower_first = 0,
-			.count = 4294967295U,
+		{
+			.extent[0] = {
+				.first = 0,
+				.lower_first = 0,
+				.count = 4294967295U,
+			},
 		},
 	},
 	.gid_map = {
 		.nr_extents = 1,
-		.extent[0] = {
-			.first = 0,
-			.lower_first = 0,
-			.count = 4294967295U,
+		{
+			.extent[0] = {
+				.first = 0,
+				.lower_first = 0,
+				.count = 4294967295U,
+			},
 		},
 	},
 	.projid_map = {
 		.nr_extents = 1,
-		.extent[0] = {
-			.first = 0,
-			.lower_first = 0,
-			.count = 4294967295U,
+		{
+			.extent[0] = {
+				.first = 0,
+				.lower_first = 0,
+				.count = 4294967295U,
+			},
 		},
 	},
 	.count = ATOMIC_INIT(3),