bpf: Reject writes through untrusted BTF pointers

check_ptr_to_btf_access() lets program-type btf_struct_access callbacks
validate writes before the default BTF access path rejects non-read
accesses. That bypasses the read-only policy for untrusted BTF pointers
created by helpers such as bpf_rdonly_cast().

Reject non-read accesses through PTR_UNTRUSTED BTF pointers at the
common entry point, before the callback branch to handle all cases.

Fixes: 282de143ead9 ("bpf: Introduce allocated objects support")
Signed-off-by: Nicholas Dudar <main.kalliope@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>