netns: use a spin_lock to protect nsid management

netns: use a spin_lock to protect nsid management

Before this patch, nsid were protected by the rtnl lock. The goal of this
patch is to be able to find a nsid without needing to hold the rtnl lock.

The next patch will introduce a netlink socket option to listen to all
netns that have a nsid assigned into the netns where the socket is opened.
Thus, it's important to call rtnl_net_notifyid() outside the spinlock, to
avoid a recursive lock (nsid are notified via rtnl). This was the main
reason of the previous patch.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Orabug: 23634951
(cherry picked from commit 95f38411df055a0ecefe3a3d119d98241087d5ca)
Signed-off-by: Cathy Zhou <Cathy.Zhou@Oracle.COM>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>