[NET]: Move some core sock setup into sk_prot_alloc

The security_sk_alloc() and the module_get is a part of the
object allocations - move it in the proper place.

Note, that since we do not reset the newly allocated sock
in the sk_alloc() (memset() is removed with the previous
patch) we can safely do this.

Also fix the error path in sk_prot_alloc() - release the security
context if needed.

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