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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master

Details
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https://git.postgresql.org/pg/commitdiff/6c05ef5729c04d47400660cd994305bc44a3c757

Modified Files
--------------
contrib/amcheck/verify_nbtree.c                               | 2 +-
src/test/modules/nbtree/Makefile                              | 2 +-
src/test/modules/nbtree/expected/nbtree_incomplete_splits.out | 7 +++++++
src/test/modules/nbtree/sql/nbtree_incomplete_splits.sql      | 7 +++++++
4 files changed, 16 insertions(+), 2 deletions(-)