xfrm_user: fix freeing of xfrm states on acquire

Commit 565f0fa9 ("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: 565f0fa9 ("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>