pgsql: Fix bug in detecting concurrent page splits in GiST insert

Fix bug in detecting concurrent page splits in GiST insert

In commit 9eb5607e699, I got the condition on checking for split or
deleted page wrong: I used && instead of ||. The comment correctly said
"concurrent split _or_ deletion".

As a result, GiST insertion could miss a concurrent split, and insert to
wrong page. Duncan Sands demonstrated this with a test script that did a
lot of concurrent inserts.

Backpatch to v12, where this was introduced. REINDEX is required to fix
indexes that were affected by this bug.

Backpatch-through: 12
Reported-by: Duncan Sands
Discussion: https://www.postgresql.org/message-id/a9690483-6c6c-3c82-c8ba-dc1a40848f11%40deepbluecap.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/0326635dd484fe1d5272dd0c551b78e3f779acb4

Modified Files
--------------
src/backend/access/gist/gist.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)