xfs: add a xfs_agino_to_ino helper

xfs: add a xfs_agino_to_ino helper

Source kernel commit: 6abd82ab6ea48430c13caebaad436ca6b5f2c34d

Add a helpers to convert an agino to an ino 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>