MP-safe IPsec SPD

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

This is a patch to make IPsec SPD MP-safe:
  http://www.netbsd.org/~ozaki-r/mpsafe-ipsec-spd.diff

Please check the locking notes in the patch to know
how I MP-ify SPD.

This is the first time for me to use localcount(9)
so please comment/suggest me on it.

Thanks,
  ozaki-r