nfsd4: clean up validate_stateid

The logic here is better expressed with a switch statement.

While we're here, CLOSED stateids (or stateids of an unkown type--which
would indicate a server bug) should probably return nfserr_bad_stateid,
though this behavior shouldn't affect any non-buggy client.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>