eventpoll: use hlist_is_singular_node() in __ep_remove()

Replace the open-coded "epi is the only entry in file->f_ep" check
with hlist_is_singular_node(). Same semantics, and the helper avoids
the head-cacheline access in the common false case.

Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-1-2470f9eec0f5@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Author: brauner

fs/eventpoll.c

@@ -856,7 +856,7 @@ static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
 
 	to_free = NULL;
 	head = file->f_ep;
-	if (head->first == &epi->fllink && !epi->fllink.next) {
+	if (hlist_is_singular_node(&epi->fllink, head)) {
 		/* See eventpoll_release() for details. */
 		WRITE_ONCE(file->f_ep, NULL);
 		if (!is_file_epoll(file)) {