bcachefs: convert eytzinger0_find to be 1-based

bcachefs: convert eytzinger0_find to be 1-based

Several of the algorithms on eytzinger trees are implemented in terms of
the eytzinger0 primitives.  However, those algorithms can just as easily
be expressed in terms of the eytzinger1 primitives, and that leads to
better and easier to understand code.  Start by converting
eytzinger0_find().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>