send_fragment(): Try including PIM header as well in fragments ...

RFC4601 seems to suggest that each fragment should consist of not only
the IP header, but also a PIM header, before the inner/encapsulated IP
packet:

    "If a multicast packet is fragmented on the way into the Register
     Tunnel, each fragment is encapsulated individually so it contains
     IP, PIM, and inner IP headers." -- RFC4601 section 4.4.1

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>