xfs: return an offset instead of a pointer from xlog_align

This simplifies both the helper and the callers.  We lost a bit of
size sanity checking, but that is already covered by KASAN if needed.

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>