libbpf: Remove unnecessary null check in kernel_supports()

After recent changes, Coverity complained about inconsistent null checks
in kernel_supports() function:

    kernel_supports(const struct bpf_object *obj, ...)
    [...]
    // var_compare_op: Comparing obj to null implies that obj might be null
    if (obj && obj->gen_loader)
        return true;

    // var_deref_op: Dereferencing null pointer obj
    if (obj->token_fd)
        return feat_supported(obj->feat_cache, feat_id);
    [...]

- The original null check was introduced by commit [0], which introduced
  a call `kernel_supports(NULL, ...)` in function bump_rlimit_memlock();
- This call was refactored to use `feat_supported(NULL, ...)` in commit [1].

Looking at all places where kernel_supports() is called:

- There is either `obj->...` access before the call;
- Or `obj` comes from `prog->obj` expression, where `prog` comes from
  enumeration of programs in `obj`;
- Or `obj` comes from `prog->obj`, where `prog` is a parameter to one
  of the API functions:
  - bpf_program__attach_kprobe_opts;
  - bpf_program__attach_kprobe;
  - bpf_program__attach_ksyscall.

Assuming correct API usage, it appears that `obj` can never be null when
passed to kernel_supports(). Silence the Coverity warning by removing
redundant null check.

  [0] e542f2c4 ("libbpf: Auto-bump RLIMIT_MEMLOCK if kernel needs it for BPF")
  [1] d6dd1d49 ("libbpf: Further decouple feature checking logic from bpf_object")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240131212615.20112-1-eddyz87@gmail.com

tools/lib/bpf/libbpf.c

@@ -4661,7 +4661,7 @@ bpf_object__probe_loading(struct bpf_object *obj)
 
 bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id feat_id)
 {
-	if (obj && obj->gen_loader)
+	if (obj->gen_loader)
 		/* To generate loader program assume the latest kernel
 		 * to avoid doing extra prog_load, map_create syscalls.
 		 */