libbpf: further decouple feature checking logic from bpf_object

Add feat_supported() helper that accepts feature cache instead of
bpf_object. This allows low-level code in bpf.c to not know or care
about higher-level concept of bpf_object, yet it will be able to utilize
custom feature checking in cases where BPF token might influence the
outcome.

Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231213190842.3844987-4-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

src/libbpf_internal.h

@@ -360,8 +360,11 @@ enum kern_feature_id {
 	__FEAT_CNT,
 };
 
-int probe_memcg_account(void);
+struct kern_feature_cache;
+bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id);
 bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id);
+
+int probe_memcg_account(void);
 int bump_rlimit_memlock(void);
 
 int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz);