bcachefs: Improve trace_move_extent_finish

We're currently debugging issues with rebalance, where it's not making
progress as quickly as it should be (or sometimes not at all).

Add the full data_update to the move_extent_finish tracepoint, so we can
check that the replicas we wrote match what we were supposed to do.

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