tftpd: Flush files as soon as they are fully received

On an RRQ, tftpd doesn't exit as soon as it's finished receiving a file.
Instead, it waits five seconds just in case the client didn't receive the
server's last ACK and decides to resend the final DATA packet.
Unfortunately, this created a 5 second delay from when the client thinks
it's done sending the file, and when the file is available for other
processes.

Fix this bug by closing the file as soon as receipt is finished.

PR: 157700
Reported by: Barry Mishler <barry_mishler@yahoo.com>
MFC after: 3 weeks