xfrm_user: fix freeing of xfrm states on acquire

commit 4a135e538962cb00a9667c82e7d2b9e4d7cd7177 upstream.

Commit 565f0fa902b6 ("xfrm: use a dedicated slab cache for struct
xfrm_state") moved xfrm state objects to use their own slab cache.
However, it missed to adapt xfrm_user to use this new cache when
freeing xfrm states.

Fix this by introducing and make use of a new helper for freeing
xfrm_state objects.

Fixes: 565f0fa902b6 ("xfrm: use a dedicated slab cache for struct xfrm_state")
Reported-by: Pan Bian <bianpan2016@163.com>
Cc: <stable@vger.kernel.org> # v4.18+
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 0eb390c..da588de 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -1552,6 +1552,7 @@
 		    int (*func)(struct xfrm_state *, int, void*), void *);
 void xfrm_state_walk_done(struct xfrm_state_walk *walk, struct net *net);
 struct xfrm_state *xfrm_state_alloc(struct net *net);
+void xfrm_state_free(struct xfrm_state *x);
 struct xfrm_state *xfrm_state_find(const xfrm_address_t *daddr,
 				   const xfrm_address_t *saddr,
 				   const struct flowi *fl,