bcachefs: Don't use the new_fs() bucket alloc path on an initialized fs

On a new filesystem or device we have to allocate the journal with a
bump allocator, because allocation info isn't ready yet - but when
hot-adding a device that doesn't have a journal, we don't want to use
that path.

Reported-by:  <syzbot+24a867cb90d8315cccff@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>