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

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>
Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Sun Nov  9 22:51:19 CET 2014 on sn-devel-104