btrfs: fix memory leak in update_space_info failure path

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)
Cc: <stable@vger.kernel.org> # v3.14+
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>