Revert "netfilter: Replace zero-length array with flexible-array member"

This reverts commit 9857e028 which is
commit 6daf14140129d30207ed6a0a69851fa6a3636bda upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I72ea16907de19ad38a993363ae45bab25f67d57a
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>