net: blackhole_dev: fix build warning for ethh set but not used

lib/test_blackhole_dev.c sets a variable that is never read, causing
this following building warning:

	lib/test_blackhole_dev.c:32:17: warning: variable 'ethh' set but not used [-Wunused-but-set-variable]

Remove the variable struct ethhdr *ethh, which is unused.

Fixes: 509e56b3 ("blackhole_dev: add a selftest")
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>