netlabel: Fix build problems when IPv6 is not enabled

A recent fix to the the NetLabel code caused build problem with
configurations that did not have IPv6 enabled; see below:

 netlabel_kapi.c: In function 'netlbl_cfg_unlbl_map_add':
 netlabel_kapi.c:165:4:
  error: implicit declaration of function 'netlbl_af6list_add'

This patch fixes this problem by making the IPv6 specific code conditional
on the IPv6 configuration flags as we done in the rest of NetLabel and the
network stack as a whole.  We have to move some variable declarations
around as a result so things may not be quite as pretty, but at least it
builds cleanly now.

Some additional IPv6 conditionals were added to the NetLabel code as well
for the sake of consistency.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Paul Moore <pmoore@redhat.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: David S. Miller <davem@davemloft.net>