nfsv4.1/4.2 client: fix handling of delegations for "oneopenown" mnt option

If a delegation for a file has been acquired, the "oneopenown" option
was ignored when the local open was issued. This could result in multiple
openowners/opens for a file, that would be transferred to the server
when the delegation was recalled.
This would not be serious, but could result in more than one openowner.
Since the Amazon/EFS does not issue delegations, this probably never
occurs in practice.
Spotted during code inspection.

This small patch fixes the code so that it checks for "oneopenown"
when doing client local opens on a delegation.

MFC after: 2 weeks