tftp-hpa: add error check for disk filled up

Add error check when the write-buffer is finally flushed to the file,
the caller can detect if the disk filled up (or had an i/o error) and
return a NOSAPCE nak to the other side.

Signed-off-by: Roy.Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>