netfilter: nfnetlink_cthelper: hit EBUSY on updates if size mismatches

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>