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

contrib/amcheck/verify_nbtree.c

@@ -784,7 +784,7 @@ bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
 							 errmsg("block %u is not leftmost in index \"%s\"",
 									current, RelationGetRelationName(state->rel))));
 
-				if (level.istruerootlevel && !P_ISROOT(opaque))
+				if (level.istruerootlevel && (!P_ISROOT(opaque) && !P_INCOMPLETE_SPLIT(opaque)))
 					ereport(ERROR,
 							(errcode(ERRCODE_INDEX_CORRUPTED),
 							 errmsg("block %u is not true root in index \"%s\"",