xfs: cleanup xlog_bread

Most callers of xlog_bread need to call xlog_align to get the actual offset.
Consolidate that call into the main xlog_bread and provide a _xlog_bread
for those few that don't want the actual offset.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>