NFSv4.1: Fix uninitialised variable in devicenotify

stable inclusion
from linux-4.19.230
commit e6b0f9177c43ff9ad0f1a6dd3639c383373911b7

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

[ Upstream commit b05bf5c63b326ce1da84ef42498d8e0e292e694c ]

When decode_devicenotify_args() exits with no entries, we need to
ensure that the struct cb_devicenotifyargs is initialised to
{ 0, NULL } in order to avoid problems in
nfs4_callback_devicenotify().

Reported-by:  <rtm@csail.mit.edu>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>