ICMP46 error: Clone first buffer instead of "truncating" original buffer

Previous code was walked buffer chain, effectively trying to "truncate" the chain, reset the
length of first buffer and reused that as the ICMP error message. That could have issues in cases
there were other users of the buffer chain. Update to clone the first buffer in chain, and
use that for the ICMP error message instead.

Change-Id: Ibc1a0bf2d854dae41874808c8297028ed93dd69d
Signed-off-by: Ole Troan <ot@cisco.com>