Fix nbtree maximum item size macro.

Commit dd299df8189, which made heap TID a tiebreaker nbtree index
column, introduced new rules on page space management to make suffix
truncation safe for v4+ indexes.  New pivot tuples (generated by suffix
truncation during leaf page splits) sometimes require dedicated extra
space at the end of a new leaf page high key/pivot to store a heap TID
using a special representation (a representation only used in pivot
tuples).

The definition of "1/3 of a page" was reduced by a single MAXALIGN()
quantum for v4 indexes to make sure that the final enlarged pivot tuple
always fit, even with a split point whose firstright tuple happened to
already be at the "1/3 of a page" limit (limit for non-pivot tuples).
Internal pages (which only contain pivot tuples) stuck with the original
"1/3 of a page" definition.  This scheme made it impossible for any page
split to fail to free enough space for its newitem, which is never okay.

The macro that determines whether non-pivot tuples exceed their "1/3 of
a leaf page" restriction was structured as if space was needed for all
three tuples during a leaf page split (the new pivot plus two very large
adjoining non-pivots that are separated by the split).  This was subtly
wrong, in that it accidentally relied on implementation details that
could (at least in theory) change in the future.

To fix, make the macro subtract a single MAXALIGN() quantum, once.  The
macro evaluates to exactly the same value as before in practice.  But it
no longer depends on the current layout of nbtree's special area struct.

No backpatch, since this isn't a live bug.

Author: Peter Geoghegan <pg@bowt.ie>
Reported-By: Robert Haas <robertmhaas@gmail.com>
Diagnosed-By: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CA+Tgmoa7UBxivM7f6Ocx_qbq4=ky3uXc+WZNOBcVX+kvJvWOEA@mail.gmail.com

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