btrfs: add helper to kill background process running _btrfs_stress_balance

btrfs: add helper to kill background process running _btrfs_stress_balance

Killing a background process running _btrfs_stress_balance() is not as
simple as sending a signal to the process and waiting for it to die.
Therefore we have the following logic to terminate such process:

   kill $pid
   wait $pid
   # Wait for the balance operation to finish.
   while ps aux | grep "balance start" | grep -qv grep; do
       sleep 1
   done

Since this is repeated in several test cases, move this logic to a common
helper and use it in all affected test cases. This will help to avoid
repeating the same code again several times in upcoming changes.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>