ure: improve transmit checksum offloading

Apparently, the name of the variable l4off was correct. Providing the
offset to the TCP or UDP header allows the transmit checksum offload to
work for TCP/IPv4, TCP/IPv6, UDP/IPv4, and UDP/IPv6.

Reported by: vishwin
Reviewed by: vishwin
Differential Revision: https://reviews.freebsd.org/D55187
Event: Wiesbaden Hackathon 2026

(cherry picked from commit 7266121ce985a1a895441357c20b0e9d56b4e5f5)