net: Allow userns root to control the network bridge code.

Allow an unpriviled user who has created a user namespace, and then
created a network namespace to effectively use the new network
namespace, by reducing capable(CAP_NET_ADMIN) and
capable(CAP_NET_RAW) calls to be ns_capable(net->user_ns,
CAP_NET_ADMIN), or capable(net->user_ns, CAP_NET_RAW) calls.

Allow setting bridge paramters via sysfs.

Allow all of the bridge ioctls:
BRCTL_ADD_IF
BRCTL_DEL_IF
BRCTL_SET_BRDIGE_FORWARD_DELAY
BRCTL_SET_BRIDGE_HELLO_TIME
BRCTL_SET_BRIDGE_MAX_AGE
BRCTL_SET_BRIDGE_AGING_TIME
BRCTL_SET_BRIDGE_STP_STATE
BRCTL_SET_BRIDGE_PRIORITY
BRCTL_SET_PORT_PRIORITY
BRCTL_SET_PATH_COST
BRCTL_ADD_BRIDGE
BRCTL_DEL_BRDIGE

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>