null_blk: prevent crash from bad home_node value

At module load, if the selected home_node value is greater than
the available numa nodes, the system will crash in
__alloc_pages_nodemask() due to a bad paging request.  Prevent this
user error crash by detecting the bad value, logging an error, and
setting g_home_node back to the default of NUMA_NO_NODE.

Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/null_blk_main.c

@@ -1748,6 +1748,11 @@ static int __init null_init(void)
 		return -EINVAL;
 	}
 
+	if (g_home_node != NUMA_NO_NODE && g_home_node >= nr_online_nodes) {
+		pr_err("null_blk: invalid home_node value\n");
+		g_home_node = NUMA_NO_NODE;
+	}
+
 	if (g_queue_mode == NULL_Q_RQ) {
 		pr_err("null_blk: legacy IO path no longer available\n");
 		return -EINVAL;