ANDROID: preserve CRC values in struct sk_buff due to ip_defrag_offset removal

In commit 7d056784 ("inet: inet_defrag: prevent sk release while
still in use"), the ip_defrag_offset variable was removed from the union
in struct sk_buff and moved to an internal-to-the-network-stack location
in order to resolve an issue.

This does not change the structure size, but the ABI tracking logic
doesn't appreciate the variable going away, so add the union back.

No functional changes here as no external code could ever use the
ip_defrag_offset variable as seen in the commit that changed this.

Bug: 161946584
Fixes: 7d056784 ("inet: inet_defrag: prevent sk release while still in use")
Change-Id: Id09f9398f41d9dbe9742807495d260e1f7a7a516
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>