9p: fix 9p.txt to advertise msize instead of maxdata

9p.txt advertises that maxdata mount option should be used to specify
msize, in the code though we use msize option and completely ignore
maxdata if passed

Signed-off-by: Nicolae Mogoreanu <mogoreanu@gmail.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>