key_unregister_ifnet() was erroneously decrementing the size of the
global security policy database, V_sptree, but it doesn't modify that
database. V_sptree_ifnet is for policies attached to if_ipsec
interfaces. Note also that key_register_ifnet() was not incrementing
the counter.
Move the definition of V_spd_size up to the rest of the SPD definitions.
Its inclusion among the SPD cache definitions was confusing to me.