btrfs: 'subvolume stress' tasks need to exit gracefully

The following scenario can occur when running btrfs/066,

  Task A                                Task B                     Task C

  run_test()
  - Execute _btrfs_stress_subvolume()
    in a background shell.
                                        _btrfs_stress_subvolme()
                                          ...
                                        - fork & exec "mount"
                                               	      		   Mount subvolume on directory in $TEST_DIR
  - Wait for fsstress to finish                                    do_mount()
  - kill shell process executing                                   - btrfs_mount()
    _btrfs_stress_subvolume()
    i.e. Task B.
  - Init process becomes the parent
    of "subvolume mount" task
    i.e. Task C.
  - In case subvolume is mounted
    (which is not the case),
    unmount it.
                                                                   - Complete mounting subvolume

Hence on the completion of one iteration of run_test(), the subvolume
created inside the filesystem on $SCRATCH_DEV continues to be mounted on
$TEST_DIR/$seq.mnt. Subsequent invocations of run_test() (called for
remaining Btrfs profile configs) fail during _scratch_pool_mkfs.

Instead of killing the 'subvolume stress' task this commit makes
_btrfs_stress_subvolume() to break out of the loop when a file exists
on the filesystem. The commit also makes relevant changes to other
users of _btrfs_stress_subvolume() i.e. btrfs/060, btrfs/065,
btrfs/067 & btrfs/068.

Suggested-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

common/rc

@@ -3280,9 +3280,10 @@ _btrfs_stress_subvolume()
 	local btrfs_mnt=$2
 	local subvol_name=$3
 	local subvol_mnt=$4
+	local stop_file=$5
 
 	mkdir -p $subvol_mnt
-	while true; do
+	while [ ! -e $stop_file ]; do
 		$BTRFS_UTIL_PROG subvolume create $btrfs_mnt/$subvol_name
 		$MOUNT_PROG -o subvol=$subvol_name $btrfs_dev $subvol_mnt
 		$UMOUNT_PROG $subvol_mnt