tools: bpftool: fix return value when all eBPF programs have been shown

Change the program to have a more consistent return code. Specifically,
do not make bpftool return an error code simply because it reaches the
end of the list of the eBPF programs to show.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>