CVE-2017-5342/pass correct caplen value to ether_print()

CVE-2017-5342/pass correct caplen value to ether_print()

In that function the "length" parameter means off-the-wire length, that
is, the length declared inside the outer header. The "caplen" parameter
means the amount of bytes actually available in the captured packet.

gre_print_0() and the functions modelled after it passed the value of
"length" instead of the value of "caplen", this could make ether_print()
access beyond the memory allocated for the captured packet. Brian
Carpenter had demonstrated this for the OTV case.

Fix the involved functions that call ether_print() to pass the correct
value and leave a comment to dismiss "caplen" later as its value can be
reliably derived from the other ether_print() parameters.