selftests/bpf: Use bpf_program__clone() in veristat

selftests/bpf: Use bpf_program__clone() in veristat

Replace veristat's per-program object re-opening with
bpf_program__clone().

Previously, veristat opened a separate bpf_object for every program in a
multi-program object file, iterated all programs to enable only the
target one, and then loaded the entire object.

Use bpf_object__prepare() once, then call bpf_program__clone() for each
program individually. This lets veristat load programs one at a time
from a single prepared object.

The caller now owns the returned fd and closes it after collecting stats.
Remove the special single-program fast path and the per-file early exit
in handle_verif_mode() so all files are always processed.

Split fixup_obj() into fixup_obj_maps() for object-wide map fixups that
must run before bpf_object__prepare(), and fixup_obj() for per-program
fixups (struct_ops masking, freplace type guessing) that run before each
bpf_program__clone() call.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260317-veristat_prepare-v4-2-74193d4cc9d9@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>