xfs: factor out log write ordering from xlog_cil_push_work()

So we can use it for start record ordering as well as commit record
ordering in future.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>