Push kernel lock down to net_sysctl().

Push kernel lock down to net_sysctl().

All except PF_MPLS paths are mp-safe:
- net_link_sysctl() and following net_ifiq_sysctl() only return
  EOPNOTSUPP;
- uipc_sysctl() - mp-safe atomic access to integers;
- bpf_sysctl() - mp-safe atomic access to integers;
- pflow_sysctl() - returns statistics from per-CPU counters;
- pipex_sysctl() - mp-safe atomic access to integer;

Push kernel lock down to mpls_sysctl(). sysctl_int_bounded() do copying
with local variable, so context switch is safe. No need to wire memory
or take `sysctl_lock' rwlock(9).

Keep protocols locked as they was include pages wiring. Copying will not
sleep - no network slowdown while doing it with net lock held.

ok bluhm