bpf: force BPF_F_RDONLY_PROG on insn array creation

bpf: force BPF_F_RDONLY_PROG on insn array creation

The original implementation added a hack to check_mem_access()
to prevent programs from writing into insn arrays. To get rid
of this hack, enforce BPF_F_RDONLY_PROG on map creation.

Also fix the corresponding selftest, as the error message changes
with this patch.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Link: https://lore.kernel.org/r/20251128063224.1305482-2-a.s.protopopov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>