fix RebuildMap() to retrieve 'subarray' info

fix RebuildMap() to retrieve 'subarray' info

RebuildMap falsely returns container info for member arrays.  Retrieving
the subarray and container_dev details prior to ->load_super() changes the
result from:

md127 imsm 082c6371:74b5ce03:64972e41:6b0860d5 /dev/md/imsm
md126 imsm 082c6371:74b5ce03:64972e41:6b0860d5 /dev/md/vol0

...to:

md126 /md127/0 3e03aee2:78c3c593:1e8ecaf0:eefb53ed /dev/md/vol0
md127 imsm 082c6371:74b5ce03:64972e41:6b0860d5 /dev/md/imsm

Reported-by: Ignacy Kasperowicz <ignacy.kasperowicz@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>