Flush logical mapping files with fd opened for read/write at checkpoint

The file descriptor was opened with read-only to fsync a regular file,
which would cause EBADFD errors on some platforms.

This is similar to the recent fix done by a586cc4b (which was broken by
me with 82a5649f), except that I noticed this issue while monitoring the
backend code for similar mistakes.  Backpatch to 9.4, as this has been
introduced since logical decoding exists as of b89e1510.

Author: Michael Paquier
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/20191006045548.GA14532@paquier.xyz
Backpatch-through: 9.4

src/backend/access/heap/rewriteheap.c

@@ -1280,7 +1280,8 @@ CheckPointLogicalRewriteHeap(void)
 		}
 		else
 		{
-			int			fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
+			/* on some operating systems fsyncing a file requires O_RDWR */
+			int			fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
 
 			/*
 			 * The file cannot vanish due to concurrency since this function