net: ICMPv6 Echo-Reply message was incorrectly setup

We tried to use the received Echo-Request buffer to send
the Echo-Reply. This did not work that well as then the
source link layer address was set to be the same as
destination address by the lower level L2 driver.

Jira: ZEP-1350

Change-Id: I0e7c06866d5b863f091e1d5c2a79dfcc2f23df20
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>