fix attach_kretprobe for newer kernels (#1922)

fix #1921

For newer kernels, bcc tries to fd based kuprobe
attachment. In fd based kprobe attachment,
  config1: for symbol
  config2: for symbol offset

In python API attach_kretprobe, the offset value
is not set in lib.bpf_attach_kprobe and hence it
will be a random value and eventually causing
kretprobe attachment failure.

This is not an issue for old debugfs based attachment
as the offset will not be used if it is a retprobe.

Signed-off-by: Yonghong Song <yhs@fb.com>