mdstat: fix list detach issues

mdstat: fix list detach issues

Move ent = ent->next; to while. It was outside the loop so if there
are more than 2 elements and we are looking for 3rd element it causes
infinite loop..

Fix el->next zeroing. It causes segfault in mdstat_free(). Theses
issues were not visible in my testing because I had only 2 MD devices.

Fixes: 4b3644ab4ce6 ("mdstat: Rework mdstat external arrays handling")
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>