numa: fix slab_node(MPOL_BIND)

When a node contains only HighMem memory, slab_node(MPOL_BIND)
dereferences a NULL pointer.

[ This code seems to go back all the way to commit 19770b32609b: "mm:
filter based on a nodemask as well as a gfp_mask". Which was back in
April 2008, and it got merged into 2.6.26. - Linus ]

Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Lee Schermerhorn <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: [e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

mm/mempolicy.c

···
 		(void)first_zones_zonelist(zonelist, highest_zoneidx,
 							&policy->v.nodes,
 							&zone);
-		return zone->node;
+		return zone ? zone->node : numa_node_id();
 	}
 
 	default: