Fix StoreCatalogInheritance1 to use 32bit inhseqno

For no apparent reason, this function was using a 16bit-wide inhseqno
value, rather than the correct 32 bit width which is what is stored in
the pg_inherits catalog.  This becomes evident if you try to create a
table with more than 65535 parents, because this error appears:

ERROR:  duplicate key value violates unique constraint «pg_inherits_relid_seqno_index»
DETAIL:  Key (inhrelid, inhseqno)=(329371, 0) already exists.

Needless to say, having so many parents is an uncommon situations, which
explains why this error has never been reported despite being having
been introduced with the Postgres95 1.01 sources in commit d31084e:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/commands/creatinh.c;hb=d31084e9d111#l349

Backpatch all the way back.

David Rowley noticed this while reviewing a patch of mine.
Discussion: https://postgr.es/m/CAKJS1f8Dn7swSEhOWwzZzssW7747YB=2Hi+T7uGud40dur69-g@mail.gmail.com

Author: alvherre

src/backend/commands/tablecmds.c

@@ -301,7 +301,7 @@ static void MergeConstraintsIntoExisting(Relation child_rel, Relation parent_rel
 static void StoreCatalogInheritance(Oid relationId, List *supers,
 						bool child_is_partition);
 static void StoreCatalogInheritance1(Oid relationId, Oid parentOid,
-						 int16 seqNumber, Relation inhRelation,
+						 int32 seqNumber, Relation inhRelation,
 						 bool child_is_partition);
 static int	findAttrByName(const char *attributeName, List *schema);
 static void AlterIndexNamespaces(Relation classRel, Relation rel,
@@ -2300,7 +2300,7 @@ StoreCatalogInheritance(Oid relationId, List *supers,
 						bool child_is_partition)
 {
 	Relation	relation;
-	int16		seqNumber;
+	int32		seqNumber;
 	ListCell   *entry;
 
 	/*
@@ -2341,7 +2341,7 @@ StoreCatalogInheritance(Oid relationId, List *supers,
  */
 static void
 StoreCatalogInheritance1(Oid relationId, Oid parentOid,
-						 int16 seqNumber, Relation inhRelation,
+						 int32 seqNumber, Relation inhRelation,
 						 bool child_is_partition)
 {
 	TupleDesc	desc = RelationGetDescr(inhRelation);
@@ -2356,7 +2356,7 @@ StoreCatalogInheritance1(Oid relationId, Oid parentOid,
 	 */
 	values[Anum_pg_inherits_inhrelid - 1] = ObjectIdGetDatum(relationId);
 	values[Anum_pg_inherits_inhparent - 1] = ObjectIdGetDatum(parentOid);
-	values[Anum_pg_inherits_inhseqno - 1] = Int16GetDatum(seqNumber);
+	values[Anum_pg_inherits_inhseqno - 1] = Int32GetDatum(seqNumber);
 
 	memset(nulls, 0, sizeof(nulls));