bpftool: Define a local bpf_perf_link to fix accessing its fields

When building bpftool with !CONFIG_PERF_EVENTS:

skeleton/pid_iter.bpf.c:47:14: error: incomplete definition of type 'struct bpf_perf_link'
        perf_link = container_of(link, struct bpf_perf_link, link);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:74:22: note: expanded from macro 'container_of'
                ((type *)(__mptr - offsetof(type, member)));    \
                                   ^~~~~~~~~~~~~~~~~~~~~~
tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:68:60: note: expanded from macro 'offsetof'
 #define offsetof(TYPE, MEMBER)  ((unsigned long)&((TYPE *)0)->MEMBER)
                                                  ~~~~~~~~~~~^
skeleton/pid_iter.bpf.c:44:9: note: forward declaration of 'struct bpf_perf_link'
        struct bpf_perf_link *perf_link;
               ^

&bpf_perf_link is being defined and used only under the ifdef.
Define struct bpf_perf_link___local with the `preserve_access_index`
attribute inside the pid_iter BPF prog to allow compiling on any
configs. CO-RE will substitute it with the real struct bpf_perf_link
accesses later on.
container_of() uses offsetof(), which does the necessary CO-RE
relocation if the field is specified with `preserve_access_index` - as
is the case for struct bpf_perf_link___local.

Fixes: cbdaf71f ("bpftool: Add bpf_cookie to link output")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230707095425.168126-3-quentin@isovalent.com

tools/bpf/bpftool/skeleton/pid_iter.bpf.c

@@ -15,6 +15,11 @@ enum bpf_obj_type {
 	BPF_OBJ_BTF,
 };
 
+struct bpf_perf_link___local {
+	struct bpf_link link;
+	struct file *perf_file;
+} __attribute__((preserve_access_index));
+
 struct perf_event___local {
 	u64 bpf_cookie;
 } __attribute__((preserve_access_index));
@@ -45,10 +50,10 @@ static __always_inline __u32 get_obj_id(void *ent, enum bpf_obj_type type)
 /* could be used only with BPF_LINK_TYPE_PERF_EVENT links */
 static __u64 get_bpf_cookie(struct bpf_link *link)
 {
+	struct bpf_perf_link___local *perf_link;
 	struct perf_event___local *event;
-	struct bpf_perf_link *perf_link;
 
-	perf_link = container_of(link, struct bpf_perf_link, link);
+	perf_link = container_of(link, struct bpf_perf_link___local, link);
 	event = BPF_CORE_READ(perf_link, perf_file, private_data);
 	return BPF_CORE_READ(event, bpf_cookie);
 }