net/socket: remove duplicated init code

This refactoring work has been started by David Howells in cdfbabfb
(net: Work around lockdep limitation in sockets that use sockets) but
the exact same day in 581319c5 (net/socket: use per af lockdep
classes for sk queues), Paolo Abeni added new classes.

This reduces the amount of (nearly) duplicated code and eases the
addition of new socket types.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>