media: videobuf2-dma-sg: Fix buf->vb NULL pointer dereference

Commit a4b83deb ("media: videobuf2: rework vb2_mem_ops API")
added a new vb member to struct vb2_dma_sg_buf, but it only added
code setting this to the vb2_dma_sg_alloc() function and not to the
vb2_dma_sg_get_userptr() and vb2_dma_sg_attach_dmabuf() which also
create vb2_dma_sg_buf objects.

This is causing a crash due to a NULL pointer deref when using
libcamera on devices with an Intel IPU3 (qcam app).

Fix these crashes by assigning buf->vb in the other 2 functions too,
note libcamera tests the vb2_dma_sg_get_userptr() path, the change
to the vb2_dma_sg_attach_dmabuf() path is untested.

Fixes: a4b83deb ("media: videobuf2: rework vb2_mem_ops API")
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/common/videobuf2/videobuf2-dma-sg.c

@@ -241,6 +241,7 @@ static void *vb2_dma_sg_get_userptr(struct vb2_buffer *vb, struct device *dev,
 	buf->offset = vaddr & ~PAGE_MASK;
 	buf->size = size;
 	buf->dma_sgt = &buf->sg_table;
+	buf->vb = vb;
 	vec = vb2_create_framevec(vaddr, size);
 	if (IS_ERR(vec))
 		goto userptr_fail_pfnvec;
@@ -642,6 +643,7 @@ static void *vb2_dma_sg_attach_dmabuf(struct vb2_buffer *vb, struct device *dev,
 	buf->dma_dir = vb->vb2_queue->dma_dir;
 	buf->size = size;
 	buf->db_attach = dba;
+	buf->vb = vb;
 
 	return buf;
 }