Only reverify backing lun when _recreate_backing_lun

[ling-yun]

In TgtAdm, we alway do twice _verify_backing_lun whether backing lun exists or not.
If backing lun exists, it doesn't need to call _verify_backing_lun function again.
It only needs reverify whether backing lun exists or not when recreate backing lun.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/63281

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/63281
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=fe03c20ea35486ef0b4bb5eea6be7ff2c8564b7a
Submitter: Jenkins
Branch: master

commit fe03c20ea35486ef0b4bb5eea6be7ff2c8564b7a
Author: ling-yun <email address hidden>
Date: Fri Dec 20 10:55:18 2013 +0800

    Only reverify backing lun when create backing lun

    In TgtAdm, we alway do twice _verify_backing_lun whether backing lun
    exists or not.
    If backing lun exists, it doesn't need to call _verify_backing_lun
    function again.
    It only needs reverify whether backing lun exists or not when recreate
    backing lun.

    Change-Id: Ic11d778362540df307056a1adcba932e571b7d20
    Closes-Bug: #1262958