netfilter: nf_ct_ftp: fix deadlock when nat rewrite is needed

We can't use ct->lock, this is already used by the seqadj internals.
When using ftp helper + nat, seqadj will attempt to acquire ct->lock
again.

Revert back to a global lock for now.

Fixes: c783a29c

 ("netfilter: nf_ct_ftp: prefer skb_linearize")
Reported-by: Bruno de Paula Larini <bruno.larini@riosoft.com.br>
Signed-off-by: Florian Westphal <fw@strlen.de>