nfsd: allow open state ids to be revoked and then freed

Revoking state through 'unlock_filesystem' now revokes any open states
found.  When the stateids are then freed by the client, the revoked
stateids will be cleaned up correctly.

Possibly the related lock states should be revoked too, but a
subsequent patch will do that for all lock state on the superblock.

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