As a step towards per inpcb or socket locks, remove the net lock

As a step towards per inpcb or socket locks, remove the net lock
for netstat -a.  Introduce a global mutex that protects the tables
and hashes for the internet PCBs.  To detect detached PCB, set its
inp_socket field to NULL.  This has to be protected by a per PCB
mutex.  The protocol pointer has to be protected by the mutex as
netstat uses it.
Always take the kernel lock in in_pcbnotifyall() and in6_pcbnotify()
before the table mutex to avoid lock ordering problems in the notify
functions.
OK visa@