bcachefs: move: move_stats refactoring

bcachefs: move: move_stats refactoring

data_progress_list is gone - it was redundant with moving_context_list

The upcoming rebalance rewrite is going to have it using two different
move_stats objects with the same moving_context, depending on whether
it's scanning or using the rebalance_work btree - this patch plumbs
stats around a bit differently so that will work.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>