kernel: atm: pppoatm fix vc-mux connection failures

Backport a hot off the press upstream kernel ATM fix:

Preserve value of skb->truesize when accounting to vcc

"There's a hack in pskb_expand_head() to avoid adjusting skb->truesize
for certain skbs. Ideally it would cover ATM too. It doesn't. Just
stashing the accounted value and using it in atm_raw_pop() is probably
the easiest way to cope."

The issue was exposed by upstream with:

commit 14afee4b6092fde451ee17604e5f5c89da33e71e
Author: Reshetova, Elena <elena.reshetova@intel.com>
Date:   Fri Jun 30 13:08:00 2017 +0300

    net: convert sock.sk_wmem_alloc from atomic_t to refcount_t

But an earlier commit left the ticking timebomb:

158f323b9868 ("net: adjust skb->truesize in pskb_expand_head()

Sincerest thanks to Mathias Kresin <dev@kresin.me> for debugging
assistance and to David Woodhouse <dwmw2@infradead.org> for further
guidance, cajoling & patience in interpreting the debug I was giving him
and producing a fix!

Fixes FS#1567

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

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