xfrm: Fix memleak on xfrm state destroy

We leak the page that we use to create skb page fragments
when destroying the xfrm_state. Fix this by dropping a
page reference if a page was assigned to the xfrm_state.

Fixes: cac2661c53 ("esp4: Avoid skb_cow_data whenever possible")
Reported-by: JD <jdtxs00@gmail.com>
Reported-by: Paul Wouters <paul@nohats.ca>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_state.c

@@ -495,6 +495,8 @@ static void ___xfrm_state_destroy(struct xfrm_state *x)
 		x->type->destructor(x);
 		xfrm_put_type(x->type);
 	}
+	if (x->xfrag.page)
+		put_page(x->xfrag.page);
 	xfrm_dev_state_free(x);
 	security_xfrm_state_free(x);
 	xfrm_state_free(x);