bpftool: Use libbpf_bpf_link_type_str

This change switches bpftool over to using the recently introduced
libbpf_bpf_link_type_str function instead of maintaining its own string
representation for the bpf_link_type enum.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220523230428.3077108-13-deso@posteo.net