selftests/bpf: Accept mem from dynptr in helper funcs

This ensures that buffers retrieved from dynptr_data are allowed to be
passed in to helpers that take mem, like bpf_strncmp

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Link: https://lore.kernel.org/r/20230506013134.2492210-6-drosen@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>