loader: tftp: Copy the first block into the cache

tftp_open reads the first block so copy it in the cached data.
If we have more than one block (i.e. we called tftp_read before
tftp_preload) simply just reset the transfer.

Reported by: mmel
Reviewed by: mmel, tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33697