bcachefs: eytzinger self tests: loop cleanups

bcachefs: eytzinger self tests: loop cleanups

The iterator variable of eytzinger0_for_each() loops has been changed to
be locally scoped at some point, so remove variables defined outside the
loop that are now unused.  In addition and for clarity, use a different
variable inside those loops where an outside variable would be shadowed.

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