skb->truesize might be big even for a small packet.

Its even bigger after commit 87fb4b7b (net: more accurate skb
truesize) and big MTU.

We should allow queueing at least one packet per receiver, even with a
low RCVBUF setting.
Reported-by: NMichal Simek <monstr@monstr.eu>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>