bpftool: Add link bash completions

Extend bpftool's bash-completion script to handle new link command and its
sub-commands.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200429001614.1544-11-andriin@fb.com