frag6: drop the volatile keyword from frag6_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/D31528

(cherry picked from commit c17ae18080b4412435aa2fb91cd6e81dd6cd180b)