reread_part(): repeat ioctl if EBUSY

As reported in https://bugzilla.openvz.org/3081 and at
http://forum.openvz.org/index.php?t=msg&goto=51702,
sometimes BLKRRPART ioctl (which we call during ploop creation)
returns EBUSY, so the whole 'vzctl create' thing fails.

Let's not fail but retry.

https://bugzilla.openvz.org/3081

Reported-by: poiuty <poiuty@lepus.su>
Acked-by: Igor Sukhih <igor@parallels.com>
Acked-by: Maxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>