net: ipv6: fix NULL reference in handle_ra_neighbor

lladdr.addr doesn't point to any storage value when it's
handed off to the net_nbuf_read function.  This results
in a write to an undefined area of memory.

Fix this by pointing lladdr.addr to a net_linkaddr_storage
structure's byte storage array which can handle the maximum
specified length.

Change-Id: I05e0a0420b262ba1e5ac95cebe1f0d91f54878ce
Signed-off-by: Michael Scott <michael.scott@linaro.org>