NFSv4.2: Serialise O_DIRECT i/o and clone range

Ensure that all O_DIRECT reads and writes complete before cloning a file
range, so that both the source and destination are up to date.

Fixes: a5864c99 ("NFS: Do not serialise O_DIRECT reads and writes")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>