s390/numa: only set possible nodes within node_possible_map

Make sure that only those nodes appear in the node_possible_map that
may actually be used. Usually that means that the node online and
possible maps are identical. For mode "plain" we only have one node,
for mode "emu" we have "emu_nodes" nodes.

Before this the possible map included (with default config) 16 nodes
while usually only one was used. That made a couple of loops that
iterated over all possible nodes do more work than necessary.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>