Run sysctl net.inet.ip.forwarding without net lock.

Run sysctl net.inet.ip.forwarding without net lock.

The places in packet processing where ip_forwarding is evaluated
have been consolidated.  The remaining pieces in pf test, ip input,
and icmp input do not need consistent information.  If the integer
value is changed by another CPU, it is harmless.
The sysctl syscall sets the value atomically, so add atomic read
in network processing and remove the net lock in sysctl IPCTL_FORWARDING.

OK claudio@ mvs@