loop/001: verify all partitions are removed

loop/001: verify all partitions are removed

loop/001 does not test whether all partitions are removed successfully
during loop device partition scanning. As a result, the regression
introduced by 0da03cab87e6 ("loop: Fix deadlock when calling
blkdev_reread_part()") can not be detected.

The regression will generate below message in dmesg:

[  464.414043] __loop_clr_fd: partition scan of loop0 failed (rc=-22)

and leave orphan partitions like below:

- /dev/loop0p1
- /sys/block/loop0/loop0p1

This patch verifies all partitions are removed by checking if there is
/sys/block/loopX/loopXpY left. The expected number of partitions left is 0.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
[Omar: check devices and sysfs for both before and after]
Signed-off-by: Omar Sandoval <osandov@fb.com>