geneve: use the correct nlattr array in NL_SET_ERR_MSG_ATTR

IFLA_GENEVE_* attributes are in the data array, which is correctly
used when fetching the value, but not when setting the extended
ack. Because IFLA_GENEVE_MAX < IFLA_MAX, we avoid out of bounds
array accesses, but we don't provide a pointer to the invalid
attribute to userspace.

Fixes: a025fb5f ("geneve: Allow configuration of DF behaviour")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>