NFS: Ensure that writeback_single_inode() calls write_inode() when syncing

Since writeback_single_inode() checks the inode->i_state flags _before_ it
flushes out the data, we need to ensure that the I_DIRTY_DATASYNC flag is
already set. Otherwise we risk not seeing a call to write_inode(), which
again means that we break fsync() et al...

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>