socket: Don't use u8 type in uapi socket.h

Use plain 255 instead, which also avoid introducing an additional header
dependency on <linux/types.h>

Fixes: 26859240 ("txhash: Add socket option to control TX hash rethink behavior")
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Link: https://lore.kernel.org/r/20220531094345.13801-1-tklauser@distanz.ch


Signed-off-by: Jakub Kicinski <kuba@kernel.org>