Have unary pollset check to see if the existing fd is orphaned

This avoids an unnecessary upgrade to multipoller if there was no
do_work called between an orphan and a subsequent add. Additionally, it
avoids the need for epoll based multipoller to check for this case in
its upgrade code by ensuring all existing fds are valid at upgrade time.

src/core/iomgr/pollset_posix.c

@@ -202,8 +202,15 @@ static void unary_poll_pollset_add_fd(grpc_pollset *pollset, grpc_fd *fd) {
   if (fd == pollset->data.ptr) return;
   fds[0] = pollset->data.ptr;
   fds[1] = fd;
+  if (!grpc_fd_is_orphaned(fds[0])) {
     grpc_platform_become_multipoller(pollset, fds, GPR_ARRAY_SIZE(fds));
     grpc_fd_unref(fds[0]);
+  } else {
+    /* old fd is orphaned and we haven't cleaned it up until now, so remain a
+     * unary poller */
+    grpc_fd_unref(fds[0]);
+    pollset->data.ptr = fd;
+  }
 }
 
 static void unary_poll_pollset_del_fd(grpc_pollset *pollset, grpc_fd *fd) {