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