ANDROID: Fix abi issues in struct sk_buff

In commit 18685451 ("inet: inet_defrag: prevent sk release while
still in use"), struct sk_buff dropped an unneeded union structure.
This did not change the actual structure size or layout at all, but the
abi checker didn't like it.  So trick it by putting some __GENKSYMS__
markers in to preserve the abi correctly.

Bug: 335584858
Fixes: 18685451 ("inet: inet_defrag: prevent sk release while still in use")
Change-Id: I78ca54f9df3e03cccebc326babf1d84ccb5dc781
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>