xfs: expose xfs_bmap_local_to_extents for online repair

Allow online repair to call xfs_bmap_local_to_extents and add a void *
argument at the end so that online repair can pass its own context.

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