[UNIX]: Move the sysctl_unix_max_dgram_qlen

This will make all the sub-namespaces always use the
default value (10) and leave the tuning via sysctl
to the init namespace only.

Per-namespace tuning is coming.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>