net: fix HAVE_EFFICIENT_UNALIGNED_ACCESS typos

HAVE_EFFICIENT_UNALIGNED_ACCESS needs CONFIG_ prefix.

Also add a comment in nla_align_64bit() explaining we have
to add a padding if current skb->data is aligned, as it
certainly can be confusing.

Fixes: 35c58459 ("net: Add helpers for 64-bit aligning netlink attributes.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>