[INET]: Avoid an integer divide in rt_garbage_collect()

Since 'goal' is a signed int, compiler may emit an integer divide
to compute goal/2.

Using a right shift is OK here and less expensive.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>