nfsd: avoid race after unhash_delegation_locked()

NFS4_CLOSED_DELEG_STID and NFS4_REVOKED_DELEG_STID are similar in
purpose.
REVOKED is used for NFSv4.1 states which have been revoked because the
lease has expired.  CLOSED is used in other cases.
The difference has two practical effects.
1/ REVOKED states are on the ->cl_revoked list
2/ REVOKED states result in nfserr_deleg_revoked from
   nfsd4_verify_open_stid() and nfsd4_validate_stateid while
   CLOSED states result in nfserr_bad_stid.

Currently a state that is being revoked is first set to "CLOSED" in
unhash_delegation_locked(), then possibly to "REVOKED" in
revoke_delegation(), at which point it is added to the cl_revoked list.

It is possible that a stateid test could see the CLOSED state
which really should be REVOKED, and so return the wrong error code.  So
it is safest to remove this window of inconsistency.

With this patch, unhash_delegation_locked() always sets the state
correctly, and revoke_delegation() no longer changes the state.

Also remove a redundant test on minorversion when
NFS4_REVOKED_DELEG_STID is seen - it can only be seen when minorversion
is non-zero.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>