blackhole_dev: add a selftest

Since this is not really a device with all capabilities, this test
ensures that it has *enough* to make it through the data path
without causing unwanted side-effects (read crash!).

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/Kconfig.debug

@@ -1909,6 +1909,15 @@ config TEST_BPF
 
 	  If unsure, say N.
 
+config TEST_BLACKHOLE_DEV
+	tristate "Test blackhole netdev functionality"
+	depends on m && NET
+	help
+	  This builds the "test_blackhole_dev" module that validates the
+	  data path through this blackhole netdev.
+
+	  If unsure, say N.
+
 config FIND_BIT_BENCHMARK
 	tristate "Test find_bit functions"
 	help