bpftool: Generate skeleton for global percpu data

Enhance bpftool to generate skeletons that properly handle global percpu
variables. The generated skeleton now includes a dedicated structure for
percpu data, allowing users to initialize and access percpu variables more
efficiently.

For global percpu variables, the skeleton now includes a nested
structure, e.g.:

struct test_global_percpu_data {
	struct bpf_object_skeleton *skeleton;
	struct bpf_object *obj;
	struct {
		struct bpf_map *percpu;
	} maps;
	// ...
	struct test_global_percpu_data__percpu {
		int data;
		char run;
		struct {
			char set;
			int i;
			int nums[7];
		} struct_data;
		int nums[7];
	} *percpu;

	// ...
};

  * The "struct test_global_percpu_data__percpu *percpu" points to
    initialized data, which is actually "maps.percpu->mmaped".
  * Before loading the skeleton, updating the
    "struct test_global_percpu_data__percpu *percpu" modifies the initial
    value of the corresponding global percpu variables.
  * After loading the skeleton, "maps.percpu->mmaped" has been marked as
    read-only in libbpf. If users want to update the global percpu
    variables, they have to update the "maps.percpu" map instead.
  * For lightweight skeleton, "lskel->percpu" will be protected by
    "mprotect(p, sz, PROT_READ)".
  * For subskeleton, those variables of global percpu data will be
    skipped.

Assisted-by: Codex:gpt-5.5-xhigh
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20260813152324.97937-7-leon.hwang@linux.dev

src/skel_internal.h

@@ -131,8 +131,10 @@ static inline void skel_free_map_data(void *p, __u64 addr, size_t sz)
 {
 	if (addr != ~0ULL)
 		kvfree(p);
-	/* When addr == ~0ULL the 'p' points to
-	 * ((struct bpf_array *)map)->value. See skel_finalize_map_data.
+	/*
+	 * When addr == ~0ULL the init buffer has already been released.
+	 * For skel_finalize_map_data(), 'p' points to
+	 * ((struct bpf_array *)map)->value.
 	 */
 }
 
@@ -170,6 +172,15 @@ out:
 	return addr;
 }
 
+static inline int skel_protect_map_data(void *p, __u64 *init_val, size_t sz)
+{
+	(void)sz;
+
+	kvfree(p);
+	*init_val = ~0ULL;
+	return 0;
+}
+
 #else
 
 static inline void *skel_alloc(size_t size)
@@ -208,6 +219,15 @@ static inline void *skel_finalize_map_data(__u64 *init_val, size_t mmap_sz, int
 		return NULL;
 	return addr;
 }
+
+static inline int skel_protect_map_data(void *p, __u64 *init_val, size_t sz)
+{
+	(void)init_val;
+
+	if (mprotect(p, sz, PROT_READ))
+		return -errno;
+	return 0;
+}
 #endif
 
 static inline int skel_closenz(int fd)