udplite: fix checksum computation on the sender side

Don't fill the fields of the UDP/IP header not used for the
checksum computation before performing the checksum computation.

Reviewed by: glebius
Differential Revision: https://reviews.freebsd.org/D42275

(cherry picked from commit aa64a8f5c35c13b1c325f1a4597c987a37bec5da)