do_pin_fd() will already print out an error message if something
goes wrong.  Printing another error is unnecessary and will break
JSON output, since error messages are full objects:

$ bpftool -jp prog load tracex1_kern.o /sys/fs/bpf/a
{
    "error": "can't pin the object (/sys/fs/bpf/a): File exists"
},{
    "error": "failed to pin program"
}

Fixes: 49a086c2 ("bpftool: implement prog load command")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>