Reuse the json_writer API introduced in an earlier commit to make
bpftool able to generate JSON output on `bpftool prog show *` commands.
A new printing function is created to be passed as an argument to the
disassembler.

Similarly to plain output, opcodes are printed on request.

Outputs from sample programs have been successfully tested against a
JSON validator.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>