sfc: Pad packets to 33 bytes to prevent TX packet parser lockup

The packet parser used in the TX data path for locating checksum
fields can lose synchronisation with the TX queue manager when
handling packets that look like IPv4 but are too short (17-32 bytes).
Work around this by padding to 33 bytes.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>