NFS: Serialise O_DIRECT i/o and truncate()

Ensure that all O_DIRECT reads and writes are complete, and prevent the
initiation of new i/o until the setattr operation that will truncate the
file is complete.

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