block: fix module reference leakage from bdev_open_by_dev error path

In the Linux kernel, the following vulnerability has been resolved:

block: fix module reference leakage from bdev_open_by_dev error path

At the time bdev_may_open() is called, module reference is grabbed

already, hence module reference should be released if bdev_may_open()

failed.

This problem is found by code review.