loader: tftp: Don't let tftp timeout

When we load a kernel or module we open/close it a few times.
Since we're using the same port number each time and that we requested
the same file the ACK that we send are valid on the server side and the
server send us the file multiple times.
This makes tftp loading time very inconsistant due to the UDP "flood" that
we have to process.

Reviewed by: imp, tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33407