datapath: Fix ODP_PORT_GROUP_GET implementation.

datapath: Fix ODP_PORT_GROUP_GET implementation.

The final argument to do_get_port_group() is supposed to be a user pointer
to the number of ports, to be updated with put_user(), but it was actually
a kernel pointer, so "ovs-dpctl dump-groups" and anything else that used
this ioctl would always fail with -EFAULT.  This commit fixes it.

Bug introduced in commit 44e05eca "datapath: Prepare to support 32-bit
compatibility ioctls" for normal ioctls and for compat ioctls at their
introduction in commit 3fbd517acf"datapath: Add 32-bit compatibility
ioctls."