[NETLINK]: Remove bogus BUG_ON

Remove bogus BUG_ON(mutex_is_locked(nlk_sk(sk)->cb_mutex)), when the
netlink_kernel_create caller specifies an external mutex it might
validly be locked.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>