vfio-mdev: fix non-standard ioctl return val causing i386 build fail

What appears to be a copy and paste error from the line above gets
the ioctl a ssize_t return value instead of the traditional "int".

The associated sample code used "long" which meant it would compile
for x86-64 but not i386, with the latter failing as follows:

CC [M] samples/vfio-mdev/mtty.o
samples/vfio-mdev/mtty.c:1418:20: error: initialization from incompatible pointer type [-Werror=incompatible-pointer-types]
.ioctl = mtty_ioctl,
^
samples/vfio-mdev/mtty.c:1418:20: note: (near initialization for ‘mdev_fops.ioctl’)
cc1: some warnings being treated as errors

Since in this case, vfio is working with struct file_operations; as such:

long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);

...and so here we just standardize on long vs. the normal int that user
space typically sees and documents as per "man ioctl" and similar.

Fixes: 9d1a546c53b4 ("docs: Sample driver to demonstrate how to use Mediated device framework.")
Cc: Kirti Wankhede <[e-mail]>
Cc: Neo Jia <[e-mail]>
Cc: [e-mail]
Signed-off-by: Paul Gortmaker <[e-mail]>
Signed-off-by: Alex Williamson <[e-mail]>

include/linux/mdev.h

···
 			size_t count, loff_t *ppos);
 	ssize_t (*write)(struct mdev_device *mdev, const char __user *buf,
 			 size_t count, loff_t *ppos);
-	ssize_t (*ioctl)(struct mdev_device *mdev, unsigned int cmd,
+	long	(*ioctl)(struct mdev_device *mdev, unsigned int cmd,
 			 unsigned long arg);
 	int	(*mmap)(struct mdev_device *mdev, struct vm_area_struct *vma);
 };