farsync: use DEFINE_SPINLOCK() for spinlock

spinlock can be initialized automatically with DEFINE_SPINLOCK()
rather than explicitly calling spin_lock_init().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Liu Jian <liujian56@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>