[NETFILTER]: nf_conntrack: simplify protocol locking

Now that we don't use nf_conntrack_lock anymore but a single mutex for
all protocol handling, no need to release and grab it again for sysctl
registration.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>