io_uring: add support for IORING_OP_OPENAT

This works just like openat(2), except it can be performed async. For
the normal case of a non-blocking path lookup this will complete
inline. If we have to do IO to perform the open, it'll be done from
async context.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -34,6 +34,7 @@ struct io_uring_sqe {
 		__u32		timeout_flags;
 		__u32		accept_flags;
 		__u32		cancel_flags;
+		__u32		open_flags;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	union {
@@ -77,6 +78,7 @@ enum {
 	IORING_OP_LINK_TIMEOUT,
 	IORING_OP_CONNECT,
 	IORING_OP_FALLOCATE,
+	IORING_OP_OPENAT,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,