bcachefs: gc shouldn't care about owned_by_allocator

bcachefs: gc shouldn't care about owned_by_allocator

The owned_by_allocator field is a purely in memory thing, even if/when
we bring back GC at runtime there's no need for it to be recalculating
this field. This is prep work for pulling it out of struct bucket, and
eventually getting rid of the bucket array.

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