frag6: Avoid a possible integer overflow in fragment handling

Reviewed by: kp, markj, bz
Approved by: so
Security: FreeBSD-SA-23:06.ipv6
Security: CVE-2023-3107

(cherry picked from commit ff3d1a3f9d71e706f320f51bae258e4e1a51b388)
(cherry picked from commit 9515f04fe3b12b9e6ef6c802b647dd4cbdba621b)