imsm: use rounded size for metadata initialization

imsm: use rounded size for metadata initialization

Array size is rounded to the nearest MB, however number of data stripes
and blocks per disk are calculated using size passed by the user. If
given size is not aligned, there is a mismatch. It's not possible to
assemble raid0 migrated to raid5 since raid5 arrays use number of data
stripes to calculate array size.

Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@gmail.com>