packet: add PACKET_RESERVE sockopt
Add new sockopt to reserve some headroom in the mmaped ring frames in front of the packet payload. This can be used f.i. when the VLAN header needs to be (re)constructed to avoid moving the entire payload. Signed-off-by: Patrick McHardy <kaber@trash.net> Signed-off-by: David S. Miller <davem@davemloft.net>
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