btrfs: add helper to stop background process running _btrfs_stress_subvolume

btrfs: add helper to stop background process running _btrfs_stress_subvolume

We have this logic to stop a process running _btrfs_stress_subvolume()
spread in several test cases:

   touch $stop_file
   wait $subvol_pid

Add a helper to encapsulate that logic and also remove the stop file after
the process terminated as there's no point having it around anymore.

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>