xfstests: btrfs/276 - stop all fsstress before exiting

xfstests: btrfs/276 - stop all fsstress before exiting

Tests after 276 were failing because the background fsstress
hadn't quit prior to exit, devices couldn't be unmounted, etc.

Just use the same trick as generic/068 does, and use
a tmpfile to control whether the background loop keeps
running.

Also, no need to umount scratch at cleanup time, the scripts
do that for us.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Jan Schmidt list.xfs@jan-o-sch.net
Signed-off-by: Rich Johnston <rjohnston@sgi.com>