bonding: Use a per netns implementation of /sys/class/net/bonding_masters.

This fixes a network namespace misfeature that bonding_masters looked at
current instead of the remembering the context where in which
/sys/class/net/bonding_masters was opened in to see which network
namespace to act upon.

This removes the need for sysfs to handle tagged directories with
untagged members allowing for a conceptually simpler sysfs
implementation.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>