ipv4: fix a dst leak in tunnels

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: 7d442fab0a67 ("ipv4: Cache dst in tunnels")
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Cong Wang <[email protected]>
Cc: Tom Herbert <[email protected]>
Cc: Maciej Żenczykowski <[email protected]>
Signed-off-by: David S. Miller <[email protected]>