xen-blkback: do not leak mode property

"be->mode" is obtained from xenbus_read(), which does a kmalloc() for
the message body. The short string is never released, so do it along
with freeing "be" itself, and make sure the string isn't kept when
backend_changed() doesn't complete successfully (which made it
desirable to slightly re-structure that function, so that the error
cleanup can be done in one place).

Reported-by: Olaf Hering <olaf@aepfle.de>
CC: stable@vger.kernel.org
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>