tools: bpftool: add completion for bpftool prog "loadall"

Bash completion for proposing the "loadall" subcommand is missing. Let's
add it to the completion script.

Add a specific case to propose "load" and "loadall" for completing:

    $ bpftool prog load
                       ^ cursor is here

Otherwise, completion considers that $command is in load|loadall and
starts making related completions (file or directory names, as the
number of words on the command line is below 6), when the only suggested
keywords should be "load" and "loadall" until one has been picked and a
space entered after that to move to the next word.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>