NFS: Optimise away the close-to-open GETATTR when we have NFSv4 OPEN

NFSv4 should not need to perform an extra close-to-open GETATTR as part
of the process of looking up a regular file, since the OPEN call will
do that for us.

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