imsm: continue resync on 3-disk RAID10

imsm: continue resync on 3-disk RAID10

If RAID10 gets degraded during resync and is stopped, it doesn't continue
resync after automatic assemble and it is reported to be in sync. Resync
is blocked because the disk is missing. It should not happen for RAID10 as
it can still continue with 3 disks.

Count missing disks. Block resync only if number of missing disks exceeds
limit for given RAID level (only different for RAID10). Check if the
disk under recovery is present. If not, resync should be allowed to run.

Signed-off-by: Maksymilian Kunt <maksymilian.kunt@intel.com>
Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>