Convert unmapped mbufs before computing checksums in IPsec.

This is similar to the logic used in ip_output() to convert mbufs
prior to computing checksums. Unmapped mbufs can be sent when using
sendfile() over IPsec or using KTLS over IPsec.

Reported by: Sony Arpita Das @ Chelsio QA
Reviewed by: np
Sponsored by: Chelsio
Differential Revision: https://reviews.freebsd.org/D28187