NFSv4: Fix a race when updating an open_stateid

If we're replacing an old stateid which has a different 'other' field,
then we probably need to free the old stateid.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Tested-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>