pgstattuple: fix free space calculation

Hello,

I think that pgstattuple should use PageGetExactFreeSpace() instead of
PageGetHeapFreeSpace() or PageGetFreeSpace(). The latter two compute the
free space minus the space of a line pointer. They are used like this in
the rest of the code (heapam.c):

pagefree = PageGetHeapFreeSpace(page);

if (newtupsize > pagefree) { we need a another page for the tuple }

... so it makes sense to take the line pointer into account in this context.

But it in the pgstattuple context, I think we want the exact free space.

I have attached a patch.

Best regards,
Frédéric