netfilter: nfnetlink_cthelper: hit EBUSY on updates if size mismatches

[ Upstream commit 8971ee8b087750a23f3cd4dc55bff2d0303fd267 ]

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>