Don't bother to allocate memory and convert the log item when we
only need the block number and the length.  Just extract them directly
and call xlog_buf_readahead separately in each branch.
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>