BRIN: Handle concurrent desummarization properly

BRIN: Handle concurrent desummarization properly

If a page range is desummarized at just the right time concurrently with
an index walk, BRIN would raise an error indicating index corruption.
This is scary and unhelpful; silently returning that the page range is
not summarized is sufficient reaction.

This bug was introduced by commit 975ad4e602ff as additional protection
against a bug whose actual fix was elsewhere.  Backpatch equally.

Reported-By: Anastasia Lubennikova <[email protected]>
Diagnosed-By: Alexander Lakhin <[email protected]>
Discussion: https://postgr.es/m/2588667e-d07d-7e10-74e2-7e1e46194491@postgrespro.ru
Backpatch: 9.5 - master