Due to the bug in the df stat we need to round off
the subvolume size to align with 4Mib.
Note:- Minimum supported size in cephcsi is 1Mib,
we dont need to take care of Kib.
fixes#3240
More details at https://github.com/ceph/ceph/pull/46905
Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>