netfilter: nfnetlink_cthelper: hit EBUSY on updates if size mismatches

stable inclusion
from linux-4.19.194
commit 8aed10cd9497933ebe3fc0afe8d8ddc1af8b0d48

--------------------------------

[ Upstream commit 8971ee8b ]

The private helper data size cannot be updated. However, updates that
contain NFCTH_PRIV_DATA_LEN might bogusly hit EBUSY even if the size is
the same.

Fixes: 12f7a505 ("netfilter: add user-space connection tracking helper infrastructure")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>