selftests/bpf: Fix a btf_dump selftest failure

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 94ede2a3e913 ("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