Revert "xfrm: fix a data-race in xfrm_gen_index()"

This reverts commit 4d78b9dc which is
commit 3e4bc239 upstream.

It breaks the android ABI and if this is needed in the future, can be
brought back in an abi-safe way.

Bug: 161946584
Change-Id: I6af8ce540570c756ea9f16526c36f8815971e216
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>