bpf: Emit verbose message when prog-specific btf_struct_access rejects a write

When BPF_WRITE goes through a PTR_TO_BTF_ID register, check_ptr_to_btf_access()
delegates to env->ops->btf_struct_access(). Most implementations
(bpf_scx_btf_struct_access, tc_cls_act_btf_struct_access, etc.) return
-EACCES for disallowed fields without logging anything, so the verifier
rejects the program with an empty message. For example a scx program doing

  1: R1=trusted_ptr_task_struct()
  ...
  4: (7b) *(u64 *)(r1 +0) = r2
  verification time 83 usec
  the program is rejected

leaves the user guessing which field is off-limits.
Emit verbose message.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260615232146.5491-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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