io_uring: add socket(2) support

Supports both regular socket(2) where a normal file descriptor is
instantiated when called, or direct descriptors.

Link: https://lore.kernel.org/r/20220412202240.234207-3-axboe@kernel.dk
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -151,6 +151,7 @@ enum {
 	IORING_OP_SETXATTR,
 	IORING_OP_FGETXATTR,
 	IORING_OP_GETXATTR,
+	IORING_OP_SOCKET,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,