bcachefs: Fix evacuate_bucket tracepoint

bcachefs: Fix evacuate_bucket tracepoint

86a494c8eef9 ("bcachefs: Kill bch2_get_next_backpointer()") dropped some
things the tracepoint emitted because bch2_evacuate_bucket() no longer
looks at the alloc key - but we did want at least some of that.

We still no longer look at the alloc key so we can't report on the
fragmentation number, but that's a direct function of dirty_sectors and
a copygc concern anyways - copygc should get its own tracepoint that
includes information from the fragmentation LRU.

But we can report on the number of sectors we moved and the bucket size.

Co-developed-by: Piotr Zalewski <pZ010001011111@proton.me>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>