selftests: devlink_lib: add check for devlink device existence

If user passes devlink handle over DEVLINK_DEV variable, check if the
device exists.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20210527105515.790330-1-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/devlink_lib.sh

@@ -18,6 +18,12 @@ if [[ ! -v DEVLINK_DEV ]]; then
 
 	DEVLINK_VIDDID=$(lspci -s $(echo $DEVLINK_DEV | cut -d"/" -f2) \
 			 -n | cut -d" " -f3)
+else
+	devlink dev show $DEVLINK_DEV &> /dev/null
+	if [ $? -ne 0 ]; then
+		echo "SKIP: devlink device \"$DEVLINK_DEV\" not found"
+		exit 1
+	fi
 fi
 
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