selftests: set sysctl bc_forwarding properly in router_broadcast.sh

sysctl setting bc_forwarding for $rp2 is needed when ping_test_from h2,
otherwise the bc packets from $rp2 won't be forwarded. This patch is to
add this setting for $rp2.

Also, as ping_test_from does grep "$from" only, which could match some
unexpected output, some test case doesn't really work, like:

  # ping_test_from $h2 198.51.200.255 198.51.200.2
    PING 198.51.200.255 from 198.51.100.2 veth3: 56(84) bytes of data.
    64 bytes from 198.51.100.1: icmp_seq=1 ttl=64 time=0.336 ms

When doing grep $form (198.51.200.2), the output could still match.
So change to grep "bytes from $from" instead.

Fixes: 40f98b9a ("selftests: add a selftest for directed broadcast forwarding")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>