selftests: forwarding: multipath_eval(): Improve style

- Change the indentation of the function body from 7 spaces to one tab.
- Move initialization of weights_ratio up so that it can be referenced
  from the error message about packet difference being zero.
- Move |'s consistently to continuation line, which reindent.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>