NFSv4: Fix another bug in the close/open_downgrade code

James Drew reports another bug whereby the NFS client is now sending
an OPEN_DOWNGRADE in a situation where it should really have sent a
CLOSE: the client is opening the file for O_RDWR, but then trying to
do a downgrade to O_RDONLY, which is not allowed by the NFSv4 spec.

Reported-by: James Drews <drews@engr.wisc.edu>
Link: http://lkml.kernel.org/r/541AD7E5.8020409@engr.wisc.edu
Fixes: aee7af356e15 (NFSv4: Fix problems with close in the presence...)
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>