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