xfs: factor out a xlog_buf_readahead helper

Add a little helper to readahead a buffer if it hasn't been cancelled.

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