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>

mdmon.c

@@ -395,7 +395,7 @@ int mdmon(char *devname, int devnum, int scan, char *switchroot)
 	} else
 		pfd[0] = pfd[1] = -1;
 
-	container = malloc(sizeof(*container));
+	container = calloc(1, sizeof(*container));
 	container->devnum = devnum;
 	container->devname = devname;
 	container->arrays = NULL;