net: icmpv6: fix net_icmpv6_send_error() dropping packets

ICMPv6 error messages are not sent (on native_posix) because the first
net_pkt_write() returns an error.

pkt has just been allocated using net_pkt_alloc_with_buffer(). Trying to
write an empty packet in overwrite mode will result in an error. There
is no need to be in overwrite mode, since we want to write the LL
src/dst addresses at the beginning.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>