Use struct ipsec_level within inpcb.

Use struct ipsec_level within inpcb.

Instead of passing around u_char[4], introduce struct ipsec_level
that contains 4 ipsec levels.  This provides better type safety.
The embedding struct inpcb is globally visible for netstat(1), so
put struct ipsec_level outside of #ifdef _KERNEL.

OK deraadt@ mvs@