devmem: handle class_create() failure

I hit this when we had a bug in IDR for a few days.  Basically sysfs would
fail to create new inodes since it uses an IDR and therefore class_create
would fail.

While we are unlikely to see this fail we may as well handle it instead of
oopsing.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Reviewed-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/mem.c

@@ -908,6 +908,9 @@ static int __init chr_dev_init(void)
 		printk("unable to get major %d for memory devs\n", MEM_MAJOR);
 
 	mem_class = class_create(THIS_MODULE, "mem");
+	if (IS_ERR(mem_class))
+		return PTR_ERR(mem_class);
+
 	mem_class->devnode = mem_devnode;
 	for (minor = 1; minor < ARRAY_SIZE(devlist); minor++) {
 		if (!devlist[minor].name)