s3:smbd: fix file corruption using "write cache size != 0"

A client can:
- open a handle (h1)
- write some data to h1.
- open a 2nd handle h2 (downgrades both handles to level II)
- try to read the data on h2 (this gets old data)

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10921

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>

Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Thu Nov  6 13:09:38 CET 2014 on sn-devel-104