Fix errors in contrib/bloom index build.

 Fix errors in contrib/bloom index build.
 
 Count the number of tuples in the index honestly, instead of assuming
 that it's the same as the number of tuples in the heap.  (It might be
 different if the index is partial.)
 
 Fix counting of tuples in current index page, too.  This error would
 have led to failing to write out the final page of the index if it
 contained exactly one tuple, so that the last tuple of the relation
 would not get indexed.
 
 Back-patch to 9.6 where contrib/bloom was added.
 
 Tomas Vondra and Tom Lane
 
 Discussion: https://postgr.es/m/3b3d8eac-c709-0d25-088e-b98339a1b28a@2ndquadrant.com