drm/virtio: Fix error code in virtio_gpu_object_shmem_init()

In virtio_gpu_object_shmem_init() we are passing NULL to PTR_ERR, which
is returning 0/success.

Fix this by storing error value in 'ret' variable before assigning
shmem->pages to NULL.

Found using static analysis with Smatch.

Fixes: 64b88afb ("drm/virtio: Correct drm_gem_shmem_get_sg_table() error handling")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>