libbpf: Add kptr type tag macros to bpf_helpers.h

Include convenience definitions:
__kptr:	Unreferenced kptr
__kptr_ref: Referenced kptr

Users can use them to tag the pointer type meant to be used with the new
support directly in the map value definition.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220424214901.2743946-11-memxor@gmail.com