NFS: Make opened as optional argument in _nfs4_do_open

Check opened, only update it when non-NULL.
It's not needs define an unused value for the opened
when calling _nfs4_do_open.

v3, same as v2.

Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>