io_uring: flag new native workers with IORING_FEAT_NATIVE_WORKERS

A few reasons to do this:

- The naming of the manager and worker have changed. That's a user visible
  change, so makes sense to flag it.

- Opening certain files that use ->signal (like /proc/self or /dev/tty)
  now works, and the flag tells the application upfront that this is the
  case.

- Related to the above, using signalfd will now work as well.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -9467,7 +9467,7 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
 			IORING_FEAT_SUBMIT_STABLE | IORING_FEAT_RW_CUR_POS |
 			IORING_FEAT_CUR_PERSONALITY | IORING_FEAT_FAST_POLL |
 			IORING_FEAT_POLL_32BITS | IORING_FEAT_SQPOLL_NONFIXED |
-			IORING_FEAT_EXT_ARG;
+			IORING_FEAT_EXT_ARG | IORING_FEAT_NATIVE_WORKERS;
 
 	if (copy_to_user(params, p, sizeof(*p))) {
 		ret = -EFAULT;

include/uapi/linux/io_uring.h

@@ -262,6 +262,7 @@ struct io_uring_params {
 #define IORING_FEAT_POLL_32BITS 	(1U << 6)
 #define IORING_FEAT_SQPOLL_NONFIXED	(1U << 7)
 #define IORING_FEAT_EXT_ARG		(1U << 8)
+#define IORING_FEAT_NATIVE_WORKERS	(1U << 9)
 
 /*
  * io_uring_register(2) opcodes and arguments