namespace: change mount to mnt

mount namespace is named mnt in /proc, this change might help a lot when
accessing entries in /proc/[pid]/ns/, setns() for example.

Signed-off-by: Shuai Zhang <zs.broccoli@gmail.com>
namespace: added setns

Signed-off-by: Shuai Zhang <zs.broccoli@gmail.com>