llc: fix SAP reference counting w.r.t. socket handling

The SAP ref counter gets decremented twice when deleting a socket,
although for all but the first socket of a SAP the SAP ref counter was
incremented only once.

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>