bcachefs: btree_gc no longer uses main in-memory bucket array

bcachefs: btree_gc no longer uses main in-memory bucket array

This changes the btree_gc code to only use the second bucket array, the
one dedicated to GC. On completion, it compares what's in its in memory
bucket array to the allocation information in the btree and writes it
directly, instead of updating the main in-memory bucket array and
writing that.

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