bcachefs: Kill bch2_bucket_alloc_new_fs()

The early-early allocation path, bch2_bucket_alloc_new_fs(), is no
longer needed - and inconsistencies around new_fs_bucket_idx have been a
frequent source of bugs.

Reported-by:  <syzbot+592425844580a6598410@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>