packet: Allow packets with only a header (but no payload)

Commit 9c7077622d ("packet: make packet_snd fail on len smaller
than l2 header") added validation for the packet size in packet_snd.
This change enforces that every packet needs a header (with at least
hard_header_len bytes) plus a payload with at least one byte. Before
this change the payload was optional.

This fixes PPPoE connections which do not have a "Service" or
"Host-Uniq" configured (which is violating the spec, but is still
widely used in real-world setups). Those are currently failing with the
following message: "pppd: packet size is too short (24 <= 24)"

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1398,7 +1398,8 @@ enum netdev_priv_flags {
  *	@dma:		DMA channel
  *	@mtu:		Interface MTU value
  *	@type:		Interface hardware type
- *	@hard_header_len: Hardware header length
+ *	@hard_header_len: Hardware header length, which means that this is the
+ *			  minimum size of a packet.
  *
  *	@needed_headroom: Extra headroom the hardware may need, but not in all
  *			  cases can this be guaranteed