[NETFILTER]: nf_conntrack: switch protocol registration/unregistration to mutex

The protocol lookups done by nf_conntrack are already protected by RCU,
there is no need to keep taking nf_conntrack_lock for registration
and unregistration. Switch to a mutex.

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