NFSv4: Revert "Truncating file opens should also sync O_DIRECT writes"

We're not holding any locks, so both nfs_wb_all() and inode_dio_wait()
are unenforcible and have livelock potential. Just limit ourselves to
flushing out the data.

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