Monitor: set err on arrays not in mdstat

Monitor: set err on arrays not in mdstat

mse can be NULL when the array was not in mdstat when we read it
but existed in statelist and was recreated after reading mdstat.
In this case we set err as we can't get full update on this array
this time.
If the same array is given twice in command line it appears twice
in statelist. The first one will mark mse->devnum=INT_MAX
so the second one can't find mse. We set err on the second one as
it's not needed. Also if it becomes degraded we would look for spares
twice for the same array.

Signed-off-by: Anna Czarnowska <anna.czarnowska@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>