Add assert and log message to visibilitymap_set

Add an assert to visibilitymap_set() that the provided heap buffer is
exclusively locked, which is expected.

Also, enhance the debug logging message to specify which VM flags were
set.

Based on a related suggestion by Kirill Reshke on an in-progress
patchset.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CALdSSPhAU56g1gGVT0%2BwG8RrSWE6qW8TOfNJS1HNAWX6wPgbFA%40mail.gmail.com

src/backend/access/heap/visibilitymap.c

@@ -255,7 +255,8 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
 	uint8		status;
 
 #ifdef TRACE_VISIBILITYMAP
-	elog(DEBUG1, "vm_set %s %d", RelationGetRelationName(rel), heapBlk);
+	elog(DEBUG1, "vm_set flags 0x%02X for %s %d",
+		 flags, RelationGetRelationName(rel), heapBlk);
 #endif
 
 	Assert(InRecovery || XLogRecPtrIsInvalid(recptr));
@@ -269,6 +270,8 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
 	if (BufferIsValid(heapBuf) && BufferGetBlockNumber(heapBuf) != heapBlk)
 		elog(ERROR, "wrong heap buffer passed to visibilitymap_set");
 
+	Assert(!BufferIsValid(heapBuf) || BufferIsExclusiveLocked(heapBuf));
+
 	/* Check that we have the right VM page pinned */
 	if (!BufferIsValid(vmBuf) || BufferGetBlockNumber(vmBuf) != mapBlock)
 		elog(ERROR, "wrong VM buffer passed to visibilitymap_set");