bpf: Introduce struct bpf_map_desc in verifier

bpf: Introduce struct bpf_map_desc in verifier

Introduce struct bpf_map_desc to hold bpf_map pointer and map uid. Use
this struct in both bpf_call_arg_meta and bpf_kfunc_call_arg_meta
instead of having different representations:
 - bpf_call_arg_meta had separate map_ptr and map_uid fields
 - bpf_kfunc_call_arg_meta had an anonymous inline struct

This unifies the map fields layout across both metadata structures,
making the code more consistent and preparing for further refactoring of
map field pointer validation.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260130-verif_special_fields-v2-1-2c59e637da7d@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>