Set all_visible_according_to_vm correctly with DISABLE_PAGE_SKIPPING

It's important for 'all_visible_according_to_vm' to correctly reflect
whether the VM bit is set or not, even when we are not trusting the VM
to skip pages, because contrary to what the comment said,
lazy_scan_prune() relies on it.

If it's incorrectly set to 'false', when the VM bit is in fact set,
lazy_scan_prune() will try to set the VM bit again and dirty the page
unnecessarily. As a result, if you used DISABLE_PAGE_SKIPPING, all
heap pages were dirtied, even if there were no changes. We would also
fail to clear any VM bits that were set incorrectly.

This was broken in commit 980ae17, so backpatch to v16.

Backpatch-through: 16
Reviewed-by: Melanie Plageman, Peter Geoghegan
Discussion: https://www.postgresql.org/message-id/3df2b582-dc1c-46b6-99b6-38eddd1b2784@iki.fi

Author: hlinnaka

src/backend/access/heap/vacuumlazy.c

@@ -1136,11 +1136,7 @@ lazy_scan_skip(LVRelState *vacrel, Buffer *vmbuffer, BlockNumber next_block,
 
 		/* DISABLE_PAGE_SKIPPING makes all skipping unsafe */
 		if (!vacrel->skipwithvm)
-		{
-			/* Caller shouldn't rely on all_visible_according_to_vm */
-			*next_unskippable_allvis = false;
 			break;
-		}
 
 		/*
 		 * Aggressive VACUUM caller can't skip pages just because they are