io_uring/cmd: give inline space in request to cmds

Some io_uring commands can use some inline space in io_kiocb. We have 32
bytes in struct io_uring_cmd, expose it.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7ca779a61ee5e166e535d70df9c7f07b15d8a0ce.1726072086.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/io_uring/cmd.h

@@ -23,6 +23,15 @@ static inline const void *io_uring_sqe_cmd(const struct io_uring_sqe *sqe)
 	return sqe->cmd;
 }
 
+static inline void io_uring_cmd_private_sz_check(size_t cmd_sz)
+{
+	BUILD_BUG_ON(cmd_sz > sizeof_field(struct io_uring_cmd, pdu));
+}
+#define io_uring_cmd_to_pdu(cmd, pdu_type) ( \
+	io_uring_cmd_private_sz_check(sizeof(pdu_type)), \
+	((pdu_type *)&(cmd)->pdu) \
+)
+
 #if defined(CONFIG_IO_URING)
 int io_uring_cmd_import_fixed(u64 ubuf, unsigned long len, int rw,
 			      struct iov_iter *iter, void *ioucmd);