selftests: forwarding: devlink_lib: Add devlink_cell_size_get()

Add a helper that answers the cell size of the devlink device.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -549,3 +549,9 @@ devlink_cpu_port_get()
 
 	echo "$DEVLINK_DEV/$cpu_dl_port_num"
 }
+
+devlink_cell_size_get()
+{
+	devlink sb pool show "$DEVLINK_DEV" pool 0 -j \
+	    | jq '.pool[][].cell_size'
+}