replace U32 ifindex with S32

These are allocated in order by the kernel, if we go
over 2 billion, we've got other problems... besides
U32 to S32 conversion will work just fine anyway.

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ifbb7139e9118dd2d0c52e83c5874132e03760b30
9 files changed