netfilter: nf_conntrack: consolidate lock/unlock into unlock_wait

The spin_lock()/spin_unlock() is synchronizing on the
nf_conntrack_locks_all_lock which is equivalent to
spin_unlock_wait() but the later should be more efficient.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>