ipsec: Remove unneeded extra variable in esp4 esp_ssg_unref()

It's assigned twice, but only used to calculate the size of the
structure it points to.  Just remove it and take a sizeof the
actual structure.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>