selftests/bpf: Fix a btf_dump selftest failure

Jakub reported bpf selftest "btf_dump" failure after forwarding to
v6.11-rc1 with netdev.
  Error: #33 btf_dump
  Error: #33/15 btf_dump/btf_dump: var_data
    btf_dump_data:FAIL:find type id unexpected find type id: actual -2 < expected 0

The reason for the failure is due to
  commit 94ede2a3 ("profiling: remove stale percpu flip buffer variables")
where percpu static variable "cpu_profile_flip" is removed.

Let us replace "cpu_profile_flip" with a variable in bpf subsystem
so whenever that variable gets deleted or renamed, we can detect the
failure immediately. In this case, I picked a static percpu variable
"bpf_cgrp_storage_busy" which is defined in kernel/bpf/bpf_cgrp_storage.c.
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240802185434.1749056-1-yonghong.song@linux.dev

tools/testing/selftests/bpf/prog_tests/btf_dump.c

@@ -805,8 +805,8 @@ static void test_btf_dump_var_data(struct btf *btf, struct btf_dump *d,
 	TEST_BTF_DUMP_VAR(btf, d, NULL, str, "cpu_number", int, BTF_F_COMPACT,
 			  "int cpu_number = (int)100", 100);
 #endif
-	TEST_BTF_DUMP_VAR(btf, d, NULL, str, "cpu_profile_flip", int, BTF_F_COMPACT,
-			  "static int cpu_profile_flip = (int)2", 2);
+	TEST_BTF_DUMP_VAR(btf, d, NULL, str, "bpf_cgrp_storage_busy", int, BTF_F_COMPACT,
+			  "static int bpf_cgrp_storage_busy = (int)2", 2);
 }
 
 static void test_btf_datasec(struct btf *btf, struct btf_dump *d, char *str,