inpcb: poison several inpcb pointer in in_pcbfree()

There are few subsystems that reference inpcb and allow it to outlive
in_pcbfree(). There are no known bugs with them to unreference the
options pointers for a freed inpcb. Enforce this so that such bugs
don't appear in the future.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D43134