gro_cells: move to net/core/gro_cells.c

We have many gro cells users, so lets move the code to avoid
duplication.

This creates a CONFIG_GRO_CELLS option.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/Kconfig b/net/Kconfig
index 2f2842d..f19c0c3 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -413,6 +413,10 @@
 	bool
 	default n
 
+config GRO_CELLS
+	bool
+	default n
+
 config NET_DEVLINK
 	tristate "Network physical/parent device Netlink interface"
 	help