tcp over udp: don't copy more bytes than avaiable

When copying the data in the first mbuf to get rid of the UDP
header, use the correct length. It was copying too much (8 bytes,
the length of the UDP header).
This only applies to handling TCP over UDP packets. The support for
TCP over UDP is disabled by default.

Reported by: jtl
Reviewed by: Peter Lei
MFC after: 3 days
Sponsored by: Netflix, Inc.