Add a wrapper for testing system_info which will handle also NULL
system infos.

This will be used by the ata PIIX driver.
Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Alexandru Romanescu <a_romanescu@yahoo.co.uk>
Cc: Tejun Heo <tj@kernel.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>