bpf: Add syscall common attributes support for btf_load

bpf: Add syscall common attributes support for btf_load

BPF_BTF_LOAD can now take log parameters from both union bpf_attr and
struct bpf_common_attr, with the same merge rules as BPF_PROG_LOAD:

- if both sides provide a complete log tuple (buf/size/level) and they
  match, use it;
- if only one side provides log parameters, use that one;
- if both sides provide complete tuples but they differ, return -EINVAL.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20260512153157.28382-6-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>