Assign smack_known_web label for kernel thread's socket

Creating struct sock by sk_alloc function in various kernel subsystems
like bluetooth doesn't call smack_socket_post_create(). In such case,
received sock label is the floor('_') label and makes access deny.
Signed-off-by: jooseong lee <jooseong.lee@samsung.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>