NFSv4: Also ask for attributes when downgrading to a READ-only state

If we're downgrading from a READ+WRITE mode to a READ-only mode, then
ask for cache consistency attributes so that we avoid the revalidation
in nfs_close_context()

Fixes: 3947b74d ("NFSv4: Don't request a GETATTR on open_downgrade.")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>