bcachefs: Improve path for when btree_gc needs another pass

bcachefs: Improve path for when btree_gc needs another pass

btree_gc sometimes needs another pass when it corrects bucket generation
numbers or data types - when it finds multiple pointers of different
data types to the same bucket, it may want to keep the second one it
found.

When this happens, we now clear out bucket sector counts _without_
resetting the bucket generation/data types that we already found,
instead of resetting them to what we have in the alloc btree.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>