bpf: return PTR_TO_BTF_ID | PTR_TRUSTED from BPF kfuncs by default

bpf: return PTR_TO_BTF_ID | PTR_TRUSTED from BPF kfuncs by default

Teach the BPF verifier to treat pointers to struct types returned from
BPF kfuncs as implicitly trusted (PTR_TO_BTF_ID | PTR_TRUSTED) by
default. Returning untrusted pointers to struct types from BPF kfuncs
should be considered an exception only, and certainly not the norm.

Update existing selftests to reflect the change in register type
printing (e.g. `ptr_` becoming `trusted_ptr_` in verifier error
messages).

Link: https://lore.kernel.org/bpf/aV4nbCaMfIoM0awM@google.com/
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260113083949.2502978-1-mattbobrowski@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>