Fix race in SSI interaction with gin fast path.

The ginfast.c code previously checked for conflicts in before locking
the relevant buffer, leaving a window where a RW conflict could be
missed.  Re-order.

There was also a place where buffer ID and block number were confused
while trying to predicate-lock a page, noted by visual inspection.

Back-patch to all supported releases.  Fixes one more problem discovered
with the reproducer from bug #17949, in this case when Dmitry tried
other index types.

Reported-by: Artem Anisimov <artem.anisimov.255@gmail.com>
Reported-by: Dmitry Dolgov <9erthalion6@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/17949-a0f17035294a55e2%40postgresql.org

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