Fix rare dsa_allocate() failures due to freepage.c corruption.

In a corner case, a btree page was allocated during a clean-up operation
that could cause the tracking of the largest contiguous span of free
space to get out of whack.  That was supposed to be prevented by the use
of the "soft" flag to avoid allocating internal pages during incidental
clean-up work, but the flag was ignored in the case where the FPM was
promoted from singleton format to btree format.  Repair.

Remove an obsolete comment in passing.

Back-patch to 10, where freepage.c arrived (as support for dsa.c).

Author: Robert Haas
Diagnosed-by: Thomas Munro and Robert Haas
Reported-by: Justin Pryzby, Rick Otten, Sand Stone, Arne Roland and others
Discussion: https://postgr.es/m/CAMAYy4%2Bw3NTBM5JLWFi8twhWK4%3Dk_5L4nV5%2BbYDSPu8r4b97Zg%40mail.gmail.com

src/backend/utils/mmgr/freepage.c

@@ -1470,9 +1470,7 @@ FreePageManagerGetInternal(FreePageManager *fpm, Size npages, Size *first_page)
  * pages; if false, do it always.  Returns 0 if the soft flag caused the
  * insertion to be skipped, or otherwise the size of the contiguous span
  * created by the insertion.  This may be larger than npages if we're able
- * to consolidate with an adjacent range.  *internal_pages_used is set to
- * true if the btree allocated pages for internal purposes, which might
- * invalidate the current largest run requiring it to be recomputed.
+ * to consolidate with an adjacent range.
  */
 static Size
 FreePageManagerPutInternal(FreePageManager *fpm, Size first_page, Size npages,
@@ -1526,6 +1524,8 @@ FreePageManagerPutInternal(FreePageManager *fpm, Size first_page, Size npages,
 
 			if (!relptr_is_null(fpm->btree_recycle))
 				root = FreePageBtreeGetRecycled(fpm);
+			else if (soft)
+				return 0;		/* Should not allocate if soft. */
 			else if (FreePageManagerGetInternal(fpm, 1, &root_page))
 				root = (FreePageBtree *) fpm_page_to_pointer(base, root_page);
 			else