pfsync: fix defer timeout

Don't use a fixed number of ticks, but take hz into account so we have a
consistent timeout, regardless of what hz is set up.
Use a 20ms timeout, becaues that's what OpenBSD uses.

Reviewed by: glebius
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33242

(cherry picked from commit 93a3fa41dcff274180133712a13392d17a3e75a1)