ip_reass: do less work in ipreass_slowtimo if possible

ipreass_slowtimo avoidably uses CPU on otherwise idle boxes

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31526

(cherry picked from commit 3be3cbe06d6107486d67d8eb80480d34d084c39c)