i2c: virtio: fix completion handling

The driver currently assumes that the notify callback is only received
when the device is done with all the queued buffers.

However, this is not true, since the notify callback could be called
without any of the queued buffers being completed (for example, with
virtio-pci and shared interrupts) or with only some of the buffers being
completed (since the driver makes them available to the device in
multiple separate virtqueue_add_sgs() calls).

This can lead to incorrect data on the I2C bus or memory corruption in
the guest if the device operates on buffers which are have been freed by
the driver.  (The WARN_ON in the driver is also triggered.)

 BUG kmalloc-128 (Tainted: G        W        ): Poison overwritten
 First byte 0x0 instead of 0x6b
 Allocated in i2cdev_ioctl_rdwr+0x9d/0x1de age=243 cpu=0 pid=28
 	memdup_user+0x2e/0xbd
 	i2cdev_ioctl_rdwr+0x9d/0x1de
 	i2cdev_ioctl+0x247/0x2ed
 	vfs_ioctl+0x21/0x30
 	sys_ioctl+0xb18/0xb41
 Freed in i2cdev_ioctl_rdwr+0x1bb/0x1de age=68 cpu=0 pid=28
 	kfree+0x1bd/0x1cc
 	i2cdev_ioctl_rdwr+0x1bb/0x1de
 	i2cdev_ioctl+0x247/0x2ed
 	vfs_ioctl+0x21/0x30
 	sys_ioctl+0xb18/0xb41

Fix this by calling virtio_get_buf() from the notify handler like other
virtio drivers and by actually waiting for all the buffers to be
completed.

Fixes: 3cfc8838

 ("i2c: virtio: add a virtio i2c frontend driver")
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>