Update check_linux_raid to manage the rsync status

Hi there,

I wrote a little patch to take in consideration the resync status and rise up a warning alert. This status is not the normal state!
The original version did'nt manage this status and stayed OK.

Just to better understand the changes, this is the content of `/proc/mdstat` in resync status:

    $ cat /proc/mdstat 
    Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] [faulty] 
    md1 : active raid1 sdb1[1] sda1[0]
      10485696 blocks [2/2] [UU]
      
    md2 : active raid1 sdb2[1] sda2[0]
      1942498240 blocks [2/2] [UU]
      [==========>..........]  resync = 51.4% (999340032/1942498240) finish=299.8min speed=52431K/sec
      
    unused devices: <none>