net: modify skb_rbtree_purge to return the truesize of all purged skbs.

Tested: see the next patch is the series.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 385114de)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>