[NET]: Cleanup the allocation/freeing of the sock object

The sock object is allocated either from the generic cache with
the kmalloc, or from the proc->slab cache.

Move this logic into an isolated set of helpers and make the
sk_alloc/sk_free look a bit nicer.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>