NFSv4: Clear the open state flags if the new stateid does not match

RFC3530 and RFC5661 both prescribe that the 'opaque' field of the
open stateid returned by new OPEN/OPEN_DOWNGRADE/CLOSE calls for
the same file and open owner should match.
If this is not the case, assume that the open state has been lost,
and that we need to recover it.

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