ipv4: fix a dst leak in tunnels

This patch :

1) Remove a dst leak if DST_NOCACHE was set on dst
   Fix this by holding a reference only if dst really cached.

2) Remove a lockdep warning in __tunnel_dst_set()
    This was reported by Cong Wang.

3) Remove usage of a spinlock where xchg() is enough

4) Remove some spurious inline keywords.
   Let compiler decide for us.

Fixes: 7d442fab ("ipv4: Cache dst in tunnels")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Cong Wang <cwang@twopensource.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>