NFS: Fix a race in __nfs_list_for_each_server()

mainline inclusion
from mainline-v5.7-rc4
commit 9c07b75b
category: bugfix
bugzilla: 182252
CVE: NA

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The struct nfs_server gets put on the cl_superblocks list before
the server->super field has been initialised, in which case the
call to nfs_sb_active() will Oops. Add a check to ensure that
we skip such a list entry.

Fixes: 3c9e502b ("NFS: Add a helper nfs_client_for_each_server()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Reviewed-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>