Nested btree transactions require special care, and an upcoming patch is
going to add assertions to that effect. We don't want to be using them
unnecessarily, so this patch switches bch2_bucket_trans_early() to not
handle transaction restarts.

This patch also adds a cursor so that on transaction restart we can
continue scanning from where the previous search for an empty bucket
left off.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>