NFSv4: Replace closed stateids with the "invalid special stateid"

When decoding a CLOSE, replace the stateid returned by the server
with the "invalid special stateid" described in RFC5661, Section 8.2.3.

In nfs_set_open_stateid_locked, ignore stateids from closed state.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>