btrfs/028: kill lingering processes when test is interrupted

btrfs/028: kill lingering processes when test is interrupted

If we interrupt the test after it spawned the fsstress and balance
processes (while it's sleeping for 30 seconds * $TIME_FACTOR), we don't
kill them and they stay around for a long time, making it impossible to
unmount the scratch filesystem (failing with -EBUSY).

Fix this by adding a _cleanup function that kills the processes and
waits for them to exit.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>