wireguard: netlink: lower .maxattr for WG_CMD_GET_DEVICE

wireguard: netlink: lower .maxattr for WG_CMD_GET_DEVICE

Previously .maxattr was shared for both WG_CMD_GET_DEVICE and
WG_CMD_SET_DEVICE. Now that it is split, then we can lower it
for WG_CMD_GET_DEVICE to follow the documentation which defines
.maxattr as WGDEVICE_A_IFNAME for WG_CMD_GET_DEVICE.

$ grep -hC5 'one but not both of:' include/uapi/linux/wireguard.h
 * WG_CMD_GET_DEVICE
 * -----------------
 *
 * May only be called via NLM_F_REQUEST | NLM_F_DUMP. The command
 * should contain one but not both of:
 *
 *    WGDEVICE_A_IFINDEX: NLA_U32
 *    WGDEVICE_A_IFNAME: NLA_NUL_STRING, maxlen IFNAMSIZ - 1
 *
 * The kernel will then return several messages [...]

While other attributes weren't rejected previously, the consensus
is that nobody sends those attributes, so nothing should break.

Link: https://lore.kernel.org/r/aRyLoy2iqbkUipZW@zx2c4.com/
Suggested-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>