Add IORING_SETUP_DEFER_TASKRUN flag.

DEFER_TASKRUN (kernel 6.1+, requires SINGLE_ISSUER) defers io_uring task
work to the application thread rather than a kernel thread, reducing
cross-CPU signaling overhead on the wakeup and completion paths.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: samuel-williams-shopify

ext/io/event/selector/uring.c

@@ -267,6 +267,12 @@ VALUE IO_Event_Selector_URing_initialize(VALUE self, VALUE loop) {
 #ifdef IORING_SETUP_SINGLE_ISSUER
 	flags |= IORING_SETUP_SINGLE_ISSUER;
 #endif
+	// IORING_SETUP_DEFER_TASKRUN (kernel 6.1+, requires SINGLE_ISSUER): defer io_uring
+	// task work to the application thread rather than a kernel thread, reducing
+	// cross-CPU signaling overhead.
+#ifdef IORING_SETUP_DEFER_TASKRUN
+	flags |= IORING_SETUP_DEFER_TASKRUN;
+#endif
 
 	int result = io_uring_queue_init(URING_ENTRIES, &selector->ring, flags);
 
@@ -1290,6 +1296,9 @@ static int IO_Event_Selector_URing_supported_p(void) {
 #ifdef IORING_SETUP_SINGLE_ISSUER
 	flags |= IORING_SETUP_SINGLE_ISSUER;
 #endif
+#ifdef IORING_SETUP_DEFER_TASKRUN
+	flags |= IORING_SETUP_DEFER_TASKRUN;
+#endif
 
 	int result = io_uring_queue_init(32, &ring, flags);