netfilter: not mark a spinlock as __read_mostly

when spinlock is locked/unlocked, its elements will be changed,
so marking it as __read_mostly is not suitable.

and remove a duplicate definition of nf_conntrack_locks_all_lock
strange that compiler does not complain.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>