pgsql: Fix amcheck's handling of incomplete root splits in B-tree

Fix amcheck's handling of incomplete root splits in B-tree

When the root page is being split, it's normal that root page
according to the metapage is not marked BTP_ROOT. Fix bogus error in
amcheck about that case.

Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://www.postgresql.org/message-id/abd65090-5336-42cc-b768-2bdd66738404@iki.fi
Backpatch-through: 14

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/721b58fbec5f4a167540177659a2502383bc0c58

Modified Files
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contrib/amcheck/verify_nbtree.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)