selftests/bpf: Choose another percpu variable in bpf for btf_dump test

bpf_cgrp_storage_busy has been removed. Use bpf_bprintf_nest_level
instead. This percpu variable is also in the bpf subsystem so that
if it is removed in the future, BPF-CI will catch this type of CI-
breaking change.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260205222916.1788211-17-ameryhung@gmail.com

Author: ameryhung

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

@@ -875,8 +875,8 @@ 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, "bpf_cgrp_storage_busy", int, BTF_F_COMPACT,
-			  "static int bpf_cgrp_storage_busy = (int)2", 2);
+	TEST_BTF_DUMP_VAR(btf, d, NULL, str, "bpf_bprintf_nest_level", int, BTF_F_COMPACT,
+			  "static int bpf_bprintf_nest_level = (int)2", 2);
 }
 
 struct btf_dump_string_ctx {