frag6: do less work in frag6_slowtimo if possible

frag6_slowtimo avoidably uses CPU on otherwise idle boxes

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31528

(cherry picked from commit 8afe9481cfa382337b8a885f358fe888bddf5982)