XFRM: Fix memory leak in xfrm_state_update

Upon "ip xfrm state update ..", xfrm_add_sa() takes an extra reference on
the user-supplied SA and forgets to drop the reference when
xfrm_state_update() returns 0.  This leads to a memory leak as the
parameter SA is never freed.  This change attempts to fix the leak by
calling __xfrm_state_put() when xfrm_state_update() updates a valid SA
(err = 0).  The parameter SA is added to the gc list when the final
reference is dropped by xfrm_add_sa() upon completion.

Signed-off-by: Tushar Gohad <tgohad@mvista.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>