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/ipv4/Kconfig b/net/ipv4/Kconfig
index 6e7baaf..e30f9ca 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -187,6 +187,7 @@
 config NET_IP_TUNNEL
 	tristate
 	select DST_CACHE
+	select GRO_CELLS
 	default n
 
 config NET_IPGRE