Restore vacuum_delay_point() in GIN posting-tree leaf vacuum

Commit fd83c83d09 turned the recursive posting-tree cleanup in
ginVacuumPostingTreeLeaves() into an iterative sweep that follows the
tree's leaf pages via their rightlinks.  The recursive version called
vacuum_delay_point() while processing the tree, but that call was removed
and never re-added to the new loop.  As that commit only set out to fix a
deadlock, the removal appears to have been unintentional.

Consequently the leaf-page sweep of a single posting tree runs with no
vacuum_delay_point(), and therefore no CHECK_FOR_INTERRUPTS().  A posting
tree stores all the TIDs for one indexed key, so for a frequently
occurring key it can span a large number of leaf pages.  While such a
tree is being vacuumed the operation ignores vacuum_cost_delay and does
not respond to query cancellation or statement_timeout; an autovacuum
worker likewise cannot be interrupted mid-sweep when another backend
requests a conflicting lock.

Restore the call, placed after the current page has been unlocked and
released so that no buffer content lock is held across a potential delay
(cf. 21c27af65f).  The sibling loops in ginbulkdelete() and
ginvacuumcleanup() already call vacuum_delay_point() once per page.

Author: Paul Kim <mok03127@gmail.com>
Co-authored-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: solai v <solai.cdac@gmail.com>
Discussion: https://postgr.es/m/178447127453.110.12276981925360691905%40mail.gmail.com
Backpatch-through: 14

src/backend/access/gin/ginvacuum.c

@@ -396,6 +396,13 @@ ginVacuumPostingTreeLeaves(GinVacuumState *gvs, BlockNumber blkno)
 		if (blkno == InvalidBlockNumber)
 			break;
 
+		/*
+		 * A safe point to delay/accept interrupts: the previous page has been
+		 * unlocked and released, so we hold no buffer content lock (nor any
+		 * other LWLock) here and CHECK_FOR_INTERRUPTS() can do its job.
+		 */
+		vacuum_delay_point();
+
 		buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
 									RBM_NORMAL, gvs->strategy);
 		LockBuffer(buffer, GIN_EXCLUSIVE);