btrfs: fix memory leak in update_space_info failure path

commit 896533a7da929136d0432713f02a3edffece2826 upstream.

If we fail to add the space_info kobject, we'll leak the memory
for the percpu counter.

Fixes: 6ab0a202 (btrfs: publish allocation data in sysfs)
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>