Fix pmtu initialization

based on ip(7) man page:

To  bootstrap the path MTU discovery process on unconnected sockets, it is
possible to start with a big datagram size (up to 64K-headers bytes  long)
and let it shrink by updates of the path MTU.

but in reality, the packet has to be equal to 64K to bootstrap the pmtu process
in kernel.

pre-patch:

[root@kronosnet-node1-br0 iputils]# ./tracepath www.ubuntu.com
 1:  lilith-br0.knet.fabbione.net                          0.216ms
 2:  192.168.254.129                                      21.866ms
 3:  192.168.255.195                                     393.590ms
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     Too many hops: pmtu 65535
     Resume: pmtu 65535

post-patch:

[root@kronosnet-node1-br0 iputils]# ./tracepath www.ubuntu.com
 1?: [LOCALHOST]                                         pmtu 1500
 1:  lilith-br0.knet.fabbione.net                          0.071ms
 1:  lilith-br0.knet.fabbione.net                          0.136ms
 2:  192.168.254.129                                       1.861ms
 3:  192.168.254.129                                       1.740ms pmtu 1300
 3:  192.168.255.195                                      63.349ms
 4:  mlk-tu-ar1-tu13.intercom.it                          92.742ms
 5:  mix-br1-v1.intercom.it                               81.074ms
 6:  te0-0-2-1.252.nr11.b019138-1.mil01.atlas.cogentco.com  81.378ms
 7:  154.25.2.133                                         82.292ms
 8:  no reply
 9:  te0-0-0-12.ccr21.zrh01.atlas.cogentco.com            85.566ms
10:  be2025.ccr22.muc01.atlas.cogentco.com                91.463ms
11:  be2228.ccr41.fra03.atlas.cogentco.com                96.727ms
12:  be2305.rcr21.fra06.atlas.cogentco.com                97.462ms
13:  level3.fra06.atlas.cogentco.com                     102.176ms
14:  vlan80.csw3.Frankfurt1.Level3.net                   106.854ms asymm 17
15:  ae-82-82.ebr2.Frankfurt1.Level3.net                 107.340ms asymm 17
16:  ae-21-21.ebr1.London1.Level3.net                    106.310ms asymm 17
17:  ae-56-111.csw1.London1.Level3.net                   106.753ms asymm 16
18:  ae-126-3512.edge5.london1.Level3.net                106.753ms asymm 15
19:  SOURCE-MANA.edge5.London1.Level3.net                116.412ms asymm 16
20:  eth0.lutin.canonical.com                            108.556ms asymm 17

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
tracepath: update docs for IPv4 pmtu value

Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>