NFS: nfs_find_open_context() may only select open files

stable inclusion
from linux-4.19.198
commit e409580b19ec7923082e90c1ec9505aa0867f21d

--------------------------------

[ Upstream commit e97bc663 ]

If a file has already been closed, then it should not be selected to
support further I/O.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
[Trond: Fix an invalid pointer deref reported by Colin Ian King]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>