mdmon: fix freeing unallocated memory

mdmon: fix freeing unallocated memory

mdmon was creating a supertype struct with malloc, and thus not
necessarily getting zero-d memory.

This was causing it to segfault when called like this from the initrd:
/sbin/mdmon /proc/mdstat /sysroot

The problem was that  load_super_imsm would get called on the non-zero'd
super struct, whcih in turn calls free_super_imsm, which checks st->sb,
which should be zero but isn't and then starts freeing bogus memory.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>