pgsql: Fix _bt_allequalimage() call within critical section.

Fix _bt_allequalimage() call within critical section.

_bt_allequalimage() does complicated things, so it's not OK to call it
in a critical section. Per buildfarm failure on 'prion', which uses
-DRELCACHE_FORCE_RELEASE -DCATCACHE_FORCE_RELEASE options.

Discussion: https://www.postgresql.org/message-id/6e5bbc08-cdfc-b2b3-9e23-1a914b9850a9@iki.fi
Backpatch-through: 16, like commit ccadf73163 that introduced this

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c3b874c431bc5d1399c011e9e5ce1e30b756f930

Modified Files
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src/backend/access/nbtree/nbtree.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)