[IPV6]: Cleanups for net/ipv6/addrconf.c (kzalloc, early exit) v2

Here are some possible (and trivial) cleanups.
- use kzalloc() where possible
- invert allocation failure test like
  if (object) {
        /* Rest of function here */
  }
  to

  if (object == NULL)
        return NULL;

  /* Rest of function here */

Signed-off-by: Ingo Oeser <ioe-lkml@rameria.de>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>