sysfs: convert BUG_ON to WARN_ON

It's rude to crash the system just because the developer did something
wrong, as it prevents them from usually even seeing what went wrong.

So convert the few BUG_ON() calls that have snuck into the sysfs code
over the years to WARN_ON() to make it more "friendly".  All of these
are able to be recovered from, so it makes no sense to crash.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/sysfs/dir.c

@@ -43,7 +43,8 @@ int sysfs_create_dir_ns(struct kobject *kobj, const void *ns)
 	kuid_t uid;
 	kgid_t gid;
 
-	BUG_ON(!kobj);
+	if (WARN_ON(!kobj))
+		return -EINVAL;
 
 	if (kobj->parent)
 		parent = kobj->parent->sd;