Fix statistics breakage from bgwriter/checkpointer process split.

ForwardFsyncRequest() supposed that it could only be called in regular
backends, which used to be true; but since the splitup of bgwriter and
checkpointer, it is also called in the bgwriter.  We do not want to count
such calls in pg_stat_bgwriter.buffers_backend statistics, so fix things
so that they aren't.

(It's worth noting here that this implies an alarmingly large increase in
the expected amount of cross-process fsync request traffic, which may well
mean that the process splitup was not such a hot idea.)

src/backend/postmaster/checkpointer.c

@@ -1132,6 +1132,7 @@ ForwardFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
 	LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);
 
 	/* Count all backend writes regardless of if they fit in the queue */
+	if (!AmBackgroundWriterProcess())
 		CheckpointerShmem->num_backend_writes++;
 
 	/*
@@ -1147,6 +1148,7 @@ ForwardFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
 		 * Count the subset of writes where backends have to do their own
 		 * fsync
 		 */
+		if (!AmBackgroundWriterProcess())
 			CheckpointerShmem->num_backend_fsync++;
 		LWLockRelease(CheckpointerCommLock);
 		return false;