NFSv4.1/pnfs: Ensure that writes respect the O_SYNC flag when doing O_DIRECT

If the caller does not specify the O_SYNC flag, then it is legitimate
to return from O_DIRECT without doing a pNFS layoutcommit operation.
However if the file is opened O_DIRECT|O_SYNC then we'd better get it
right.

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