xfs: refactor xfs_iread_extents to use xfs_btree_visit_blocks

xfs_iread_extents open-codes everything in xfs_btree_visit_blocks, so
refactor the btree helper to be able to iterate only the records on
level 0, then port iread_extents to use it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>