pgsql: Fix two distinct errors in creation of GIN_INSERT_LISTPAGE xlog

Log Message:
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Fix two distinct errors in creation of GIN_INSERT_LISTPAGE xlog records.
In practice these mistakes were always masked when full_page_writes was on,
because XLogInsert would always choose to log the full page, and then
ginRedoInsertListPage wouldn't try to do anything. But with full_page_writes
off a WAL replay failure was certain.

The GIN_INSERT_LISTPAGE record type could probably be eliminated entirely
in favor of using XLOG_HEAP_NEWPAGE, but I refrained from doing that now
since it would have required a significantly more invasive patch.

In passing do a little bit of code cleanup, including making the accounting
for free space on GIN list pages more precise. (This wasn't a bug as the
errors were always in the conservative direction.)

Per report from Simon. Back-patch to 8.4 which contains the identical code.

Modified Files:
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pgsql/src/backend/access/gin:
ginfast.c (r1.3 -> r1.4)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/gin/ginfast.c?r1=1.3&r2=1.4)