[XFRM]: Do not add a state whose SPI is zero to the SPI hash.

SPI=0 is used for acquired IPsec SA and MIPv6 RO state.
Such state should not be added to the SPI hash
because we do not care about it on deleting path.

Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>