xen: irqfd: Use _IOW instead of the internal _IOC() macro

_IOC() an internal helper that we should not use in driver code.  In
particular, we got the data direction wrong here, which breaks a number
of tools, as having "_IOC_NONE" should never be paired with a nonzero
size.

Use _IOW() instead.

Fixes: f8941e6c ("xen: privcmd: Add support for irqfd")
Reported-by: Arnd Bergmann <arnd@kernel.org>
Closes: https://lore.kernel.org/all/268a2031-63b8-4c7d-b1e5-8ab83ca80b4a@app.fastmail.com/Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/599ca6f1b9dd2f0e6247ea37bee3ea6827404b6d.1697439990.git.viresh.kumar@linaro.orgSigned-off-by: Juergen Gross <jgross@suse.com>

include/uapi/xen/privcmd.h

@@ -138,6 +138,6 @@ struct privcmd_irqfd {
 #define IOCTL_PRIVCMD_MMAP_RESOURCE				\
 	_IOC(_IOC_NONE, 'P', 7, sizeof(struct privcmd_mmap_resource))
 #define IOCTL_PRIVCMD_IRQFD					\
-	_IOC(_IOC_NONE, 'P', 8, sizeof(struct privcmd_irqfd))
+	_IOW('P', 8, struct privcmd_irqfd)
 
 #endif /* __LINUX_PUBLIC_PRIVCMD_H__ */