Fix the computation of max dead tuples during the vacuum.

In commit 40d964ec, we changed the way memory is allocated for dead
tuples but forgot to update the place where we compute the maximum
number of dead tuples.  This could lead to invalid memory requests.

Reported-by: Andres Freund
Diagnosed-by: Andres Freund
Author: Masahiko Sawada
Reviewed-by: Amit Kapila and Dilip Kumar
Discussion: https://postgr.es/m/20200121060020.e3cr7s7fj5rw4lok@alap3.anarazel.de