nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

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

nfsd: drop the stateid, not the stateowner, on seqid_op replay retry

In nfs4_preprocess_seqid_op() the stateid is obtained from
nfsd4_lookup_stateid(), which holds a reference on the nfs4_stid
(sc_count) but takes no reference on the stateowner. openlockstateid()
merely casts that stid and likewise takes no reference.

When nfsd4_cstate_assign_replay() returns -EAGAIN (the replay owner is
being torn down, RP_UNHASHED) it has not taken a stateowner reference on
that path. The error handling nevertheless called
nfs4_put_stateowner(stp->st_stateowner), dropping an so_count reference
the function never acquired -- risking a stateowner refcount underflow and
use-after-free -- while leaking the sc_count reference held on the stid.
The leaked stid reference can also stall a concurrent
nfsd4_close_open_stateid() waiting for sc_count to drop.

Drop the reference actually held -- the stid -- before retrying. The
stateowner stays alive through the reference held by the stid. This mirrors
the open path in nfsd4_process_open1(), where the put balances a reference
that path explicitly holds on the stateowner.