Store the deletion horizon XID for a deleted GIN page on the right page.

Store the deletion horizon XID for a deleted GIN page on the right page.

Commit b10714080 moved the GinPageSetDeleteXid() call to a spot where
the "page" variable was pointing to the wrong page, causing the XID
to be inserted on a page that's not being deleted, thus allowing later
GinPageIsRecyclable tests to recycle the deleted page too soon.

It might be a good idea to stop using the single "page" variable for
multiple purposes in this function.  But for the moment I just moved
the GinPageSetDeleteXid() call down beside the GinPageSetDeleted()
call, which seems like a more logical place for it anyway.

Back-patch to v11, as the faulty patch was.  (Fortunately, the bug
hasn't made it into any release yet.)

Discussion: https://postgr.es/m/21620.1581098806@sss.pgh.pa.us