Set target->sbp field to valid value when sbp device is created.

The sbp_cam_detach_target can be called from sbp_post_explore function
on the first target that is not really attached and it was written with
the corresponding safety check in place to tolerate that. Unfortunately
the recent locking cleanup did add a locking assertion that tries to
dereference the target->sbp pointer unconditionally, which causes less
than desirable outcome. Since the assertion is useful, just initialize
the target sbp pointer once when sbp device is being initialized instead
of when the target is being attached. This makes assertion work in all
cases and fixes the crash on boot.