Detail: show correct raid level when the array is inactive

Detail: show correct raid level when the array is inactive

Sometimes the raid level in the output of `mdadm -D /dev/mdX` is
misleading when the array is in inactive state. Here is a testcase for
introduction.
1\ creating a raid1 device with two disks. Specify a different hostname
rather than the real one for later verfication.

node1:~ # mdadm --create /dev/md0 --homehost TESTARRAY -o -l 1 -n 2 /dev/sdb
/dev/sdc
2\ remove one of the devices and reboot
3\ show the detail of raid1 device

node1:~ # mdadm -D /dev/md127
/dev/md127:
        Version : 1.2
     Raid Level : raid0
  Total Devices : 1
    Persistence : Superblock is persistent
          State : inactive
Working Devices : 1

You can see that the "Raid Level" in /dev/md127 is raid0 now.
After step 2\ is done, the degraded raid1 device is recognized
as a "foreign" array in 64-md-raid-assembly.rules. And thus the
timer to activate the raid1 device is not triggered. The array
level returned from GET_ARRAY_INFO ioctl is 0. And the string
shown for "Raid Level" is
str = map_num(pers, array.level);
And the definition of pers is
mapping_t pers[] = {
{ "linear", LEVEL_LINEAR},
{ "raid0", 0},
{ "0", 0}
...
So the misleading "raid0" is shown in this testcase.

Changelog:
v1: don't show "Raid Level" when array is inactive
Signed-off-by: Lidong Zhong <lidong.zhong@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>