bcachefs: peek_prev_min(): Search forwards for extents, snapshots

bcachefs: peek_prev_min(): Search forwards for extents, snapshots

With extents and snapshots, for slightly different reasons, we may have
to search forwards to find a key that compares equal to iter->pos (i.e.
a key that peek_prev() should return, as it returns keys <= iter->pos).

peek_slot() does this, and is an easy way to fix this case.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>