ndisc: Fix padding error in link-layer address option.

If a natural number n exists where 2 + data_len <= 8n < 2 + data_len + pad,
post padding is not initialized correctly.

(Un)fortunately, the only type that requires pad is Infiniband,
whose pad is 2 and data_len is 20, and this logical error has not
become obvious, but it is better to fix.

Note that ndisc_opt_addr_space() handles the situation described
above correctly.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>