Add a test for offline-scrub.
The process of this test case:
1)create a filesystem with profile raid10
2)mount the filesystem, create a file in the mount point, and write
some data to the file
3)get the logical address of the file's extent data
4)get the physical address of the logical address
5)overwrite the contents in the physical address
6)use offline scrub to check and repair it

Signed-off-by: Gu Jinxiang <gujx@cn.fujitsu.com>