xfs: don't use REQ_PREFLUSH for split log writes

If we have to split a log write because it wraps the end of the log we
can't just use REQ_PREFLUSH to flush before the first log write,
as the writes might get reordered somewhere in the I/O stack.  Issue
a manual flush in that case so that the ordering of the two log I/Os
doesn't matter.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>