Our userlevel SMBwriteX call is non-standard in that it

Our userlevel SMBwriteX call is non-standard in that it
sometimes uses a 12-word write and doesn't include a pad
byte (as Windows does). Fix this so that we are identical
to Windows clients. This will make recvfile processing
much easier to detect (as we can just read a standard
writeX header length to decide).
Jeremy.
(This used to be commit 3d3d1b806aef3617abaac46daf230ed32076e2ce)