Do not allow cached open for O_RDONLY or O_WRONLY unless the file has been
previously opened in these modes.

Also Fix the calculation of the mode in nfs4_close_prepare. We should only
issue an OPEN_DOWNGRADE if we're sure that we will still be holding the
correct open modes. This may not be the case if we've been doing delegated
opens.

Finally, there is no need to adjust the open mode bit flags in
nfs4_close_done(): that has already been done in nfs4_close_prepare().
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>