bcachefs: Fix journal_entry_dev_usage_to_text() overrun

If the jset_entry_dev_usage is malformed, and too small, our nr_entries
calculation will be incorrect - just bail out.

Reported-by:  <syzbot+05d7520be047c9be86e0@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>