bpf: add support for fixed-size memory pointer returns for kfuncs

Support direct fixed-size (and for now, read-only) memory access when
kfunc's return type is a pointer to non-struct type. Calculate type size
and let BPF program access that many bytes directly. This is crucial for
numbers iterator.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230302235015.2044271-13-andrii@kernel.org


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