net: introduce a new function dst_dev_put()

This function should be called when removing routes from fib tree after
the dst gc is no longer in use.
We first mark DST_OBSOLETE_DEAD on this dst to make sure next
dst_ops->check() fails and returns NULL.
Secondly, as we no longer keep the gc_list, we need to properly
release dst->dev right at the moment when the dst is removed from
the fib/fib6 tree.
It does the following:
1. change dst->input and output pointers to dst_discard/dst_dscard_out to
   discard all packets
2. replace dst->dev with loopback interface

Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -428,6 +428,7 @@ void dst_init(struct dst_entry *dst, struct dst_ops *ops,
 	      unsigned short flags);
 void __dst_free(struct dst_entry *dst);
 struct dst_entry *dst_destroy(struct dst_entry *dst);
+void dst_dev_put(struct dst_entry *dst);
 
 static inline void dst_free(struct dst_entry *dst)
 {