ndp: Ignore entire PI if violates RFC 4862 section 5.5.3

[pouria]

Summary

Ignore prefix information update earlier in prelist_update().
If PI is invalid or autonomous bit is unset, we better to let our
SLAAC address expire.
If we don't have any previous matching prefix, better not to create new one.
Because either our router don't want us to have one anymore, or the very
RA is malicious.