xfs: add xfs_agbno_to_fsb and xfs_agbno_to_daddr helpers

xfs: add xfs_agbno_to_fsb and xfs_agbno_to_daddr helpers

Source kernel commit: 856a920ac2bbb2352ef6aa9e1e052f2e80677df7

Add helpers to convert an agbno to a daddr or fsbno based on a pag
structure.

This provides a simpler conversion and better type safety compared to the
existing code that passes the mount structure and the agno separately.

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