net: make GRO aware of skb->head_frag

GRO can check if skb to be merged has its skb->head mapped to a page
fragment, instead of a kmalloc() area.

We 'upgrade' skb->head as a fragment in itself

This avoids the frag_list fallback, and permits to build true GRO skb
(one sk_buff and up to 16 fragments), using less memory.

This reduces number of cache misses when user makes its copy, since a
single sk_buff is fetched.

This is a followup of patch "net: allow skb->head to be a page fragment"

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Ilpo Järvinen <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Maciej Żenczykowski <[email protected]>
Cc: Neal Cardwell <[email protected]>
Cc: Tom Herbert <[email protected]>
Cc: Jeff Kirsher <[email protected]>
Cc: Ben Hutchings <[email protected]>
Cc: Matt Carlson <[email protected]>
Cc: Michael Chan <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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