Failure to add offloaded policy will cause to the following
error once user will try to reload driver.

Unregister_netdevice: waiting for eth3 to become free. Usage count = 2

This was caused by xfrm_dev_policy_add() which increments reference
to net_device. That reference was supposed to be decremented
in xfrm_dev_policy_free(). However the latter wasn't called.

 unregister_netdevice: waiting for eth3 to become free. Usage count = 2
 leaked reference.
  xfrm_dev_policy_add+0xff/0x3d0
  xfrm_policy_construct+0x352/0x420
  xfrm_add_policy+0x179/0x320
  xfrm_user_rcv_msg+0x1d2/0x3d0
  netlink_rcv_skb+0xe0/0x210
  xfrm_netlink_rcv+0x45/0x50
  netlink_unicast+0x346/0x490
  netlink_sendmsg+0x3b0/0x6c0
  sock_sendmsg+0x73/0xc0
  sock_write_iter+0x13b/0x1f0
  vfs_write+0x528/0x5d0
  ksys_write+0x120/0x150
  do_syscall_64+0x3d/0x90
  entry_SYSCALL_64_after_hwframe+0x46/0xb0

Fixes: 919e43fa ("xfrm: add an interface to offload policy")
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>