net/9p: set correct stat size when sending Twstat messages

The 9P2000 Twstat message requires the size of the stat structure to be
specified. Currently the 9p code writes zero instead of the actual size.
This behavior confuses some of the file servers that check if the size is
correct.

This patch adds a new function that calculcates the stat size and puts the
value in the appropriate place in the 9P message.

Signed-off-by: Latchesar Ionkov <lucho@ionkov.net>
Reviewed-by: Eric Van Hensbergen <ericvh@gmail.com>