g up the head room is detrimental in the non-TSO
case because it makes memory that would otherwise be available to
the payload head room.  TSO doesn't need this either, all it wants
is the guarantee that there is no tail room.

So this patch fixes this by adjusting the skb_reserve call so that
exactly the requested amount (which all callers have calculated in
a precise way) is made available as tail room.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
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