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>

Documentation/filesystems/9p.txt

@@ -92,7 +92,7 @@ OPTIONS
 
   wfdno=n	the file descriptor for writing with trans=fd
 
-  maxdata=n	the number of bytes to use for 9p packet payload (msize)
+  msize=n	the number of bytes to use for 9p packet payload
 
   port=n	port to connect to on the remote server