iwlwifi: dont pull too much payload in skb head

As iwlwifi use fat skbs, it should not pull too much data in skb->head,
and particularly no tcp data payload, or splice() is slower, and TCP
coalescing is disabled. Copying payload to userland also involves at
least two copies (part from header, part from fragment)

Each layer will pull its header from the fragment as needed.

(on 64bit arches, skb_tailroom(skb) at this point is 192 bytes)

With this patch applied, I have a major reduction of collapsed/pruned
TCP packets, a nice increase of TCPRcvCoalesce counter, and overall
better Internet User experience.

Small packets are still using a fragless skb, so that page can be reused
by the driver.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>