ipsec: retire sadb_msg_reserved in order to facilitate per-cpu refcounting

[mjg]

Summary

The field is only used for debugging and the ref count would be too expensive to obtain.

[zlei]

Looks good to me except for the ABI breakage.

sys/net/pfkeyv2.h

88

Will this removal break ABI ?

sbin/setkey and lib/libipsec have reference to this data type define struct sadb_msg {}