pf: Count holes rather than fragments for reassembly

Avoid traversing the list of fragment entris to check whether the
pf(4) reassembly is complete. Instead count the holes that are
created when inserting a fragment. If there are no holes left, the
fragments are continuous.

Obtained from: OpenBSD
Differential Revision: https://reviews.freebsd.org/D17732

(cherry picked from commit 2b1c354ee6fb075953d2c3e81c8221f4115ce981)