dccp: Fix memleak in __feat_register_sp

If dccp_feat_push_change fails, we forget free the mem
which is alloced by kmemdup in dccp_feat_clone_sp_val.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: e8ef967a54f4 ("dccp: Registration routines for changing feature values")
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
