io_uring: add IORING_OP_WAITID support

This adds support for an async version of waitid(2), in a fully async
version. If an event isn't immediately available, wait for a callback
to trigger a retry.

The format of the sqe is as follows:

sqe->len		The 'which', the idtype being queried/waited for.
sqe->fd			The 'pid' (or id) being waited for.
sqe->file_index		The 'options' being set.
sqe->addr2		A pointer to siginfo_t, if any, being filled in.

buf_index, add3, and waitid_flags are reserved/unused for now.
waitid_flags will be used for options for this request type. One
interesting use case may be to add multi-shot support, so that the
request stays armed and posts a notification every time a monitored
process state change occurs.

Note that this does not support rusage, on Arnd's recommendation.

See the waitid(2) man page for details on the arguments.

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

include/uapi/linux/io_uring.h

@@ -65,6 +65,7 @@ struct io_uring_sqe {
 		__u32		xattr_flags;
 		__u32		msg_ring_flags;
 		__u32		uring_cmd_flags;
+		__u32		waitid_flags;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	/* pack this to avoid bogus arm OABI complaints */
@@ -241,6 +242,7 @@ enum io_uring_op {
 	IORING_OP_SEND_ZC,
 	IORING_OP_SENDMSG_ZC,
 	IORING_OP_READ_MULTISHOT,
+	IORING_OP_WAITID,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,