xfrm: fix gro_cells leak when remove virtual xfrm interfaces

The device gro_cells has been initialized, it should be freed,
otherwise it will be leaked

Fixes: f203b76d ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>