nfsd: prevent panic for nfsv4.0 closed files in nfs4_show_open

In the Linux kernel, the following vulnerability has been resolved:

nfsd: prevent panic for nfsv4.0 closed files in nfs4_show_open

Prior to commit 3f29cc82a84c ("nfsd: split sc_status out of
sc_type") states_show() relied on sc_type field to be of valid
type before calling into a subfunction to show content of a
particular stateid. From that commit, we split the validity of
the stateid into sc_status and no longer changed sc_type to 0
while unhashing the stateid. This resulted in kernel oopsing
for nfsv4.0 opens that stay around and in nfs4_show_open()
would derefence sc_file which was NULL.

Instead, for closed open stateids forgo displaying information
that relies of having a valid sc_file.

To reproduce: mount the server with 4.0, read and close
a file and then on the server cat /proc/fs/nfsd/clients/2/states

[ 513.590804] Call trace:
[ 513.590925] _raw_spin_lock+0xcc/0x160
[ 513.591119] nfs4_show_open+0x78/0x2c0 [nfsd]
[ 513.591412] states_show+0x44c/0x488 [nfsd]
[ 513.591681] seq_read_iter+0x5d8/0x760
[ 513.591896] seq_read+0x188/0x208
[ 513.592075] vfs_read+0x148/0x470
[ 513.592241] ksys_read+0xcc/0x178