Use correct type for catalog_xmin

Use correct type for catalog_xmin

Commit 85c17f6 mistakenly declared a variable storing catalog_xmin as
XLogRecPtr, even though catalog_xmin is a TransactionId.

This caused no functional issue, but the type was clearly incorrect.
Therefore, this commit fixes it to use the correct type TransactionId
instead, and backpatch to v17 where the issue was introduced.

Author: Imran Zaheer <imran.zhir@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CA+UBfa=mNeLt-4BFjEP4tqdDsnq+oMqqPr7fd9Wji2_9YXmQdA@mail.gmail.com