ip_reass: drop the volatile keyword from nfrags and mark with __exclusive_cache_line

The keyword adds nothing as all operations on the var are performed
through atomic_*

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

(cherry picked from commit d2b95af1c27ed51d72bef5d9f3d89860edc4fd40)