btrfs: add helper to kill background process running _btrfs_stress_remount_compress

btrfs: add helper to kill background process running _btrfs_stress_remount_compress

Killing a background process running _btrfs_stress_remount_compress() 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 $pid
    while ps aux | grep "mount.*$SCRATCH_MNT" | 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>
[ Restore 'wait $fsstress_pid' before 'kill $replace_pid' ]