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/xfrm/Kconfig b/net/xfrm/Kconfig
index bda1a13..c06d399 100644
--- a/net/xfrm/Kconfig
+++ b/net/xfrm/Kconfig
@@ -4,6 +4,7 @@
 config XFRM
        bool
        depends on NET
+       select GRO_CELLS
 
 config XFRM_ALGO
 	tristate