nfsd41: refactor nfsd4_deleg_xgrade_none_ext logic out of nfsd4_process_open2

Handle the case where the nfsv4.1 client asked to uprade or downgrade
its delegations and server returns no delegation.

In this case, op_delegate_type is set to NFS4_OPEN_DELEGATE_NONE_EXT
and op_why_no_deleg is set respectively to WND4_NOT_SUPP_{UP,DOWN}GRADE

Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>