sock: Remove the global prot_inuse counter.

The per-cpu counter for init_net is prepared in core_initcall.
The patch 7d720c3e ("percpu: add __percpu sparse annotations to net")
and d6d9ca0f ("net: this_cpu_xxx conversions") optimize the
routines. Then remove the old counter.

Cc: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Tonghao Zhang <zhangtonghao@didichuxing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>