IPv6: Calipso option length is not used properly

This field indeed indicates the length of the Compartment Bitmap but *in 32-bit words*, which means that the value of 12 means the compartment bitmap is 48 bytes long.

Issue found by boaz
Bug:10561

Change-Id: I6c6ac25d0680837594d7cc6d2e4807a60a523b80
Reviewed-on: https://code.wireshark.org/review/4697
Reviewed-by: Michael Mann <mmann78@netscape.net>