fs/fsstress: backport fix from xfstests

Original changelog:

    xfstests: fsstress should kill children tasks before exit

    It is very hard to predict runtime for fsstress. In many cases it
    is useful to give test to run a reasonable time, and then kill it.
    But currently there is no reliable way to kill test without leaving
    running children.
    This patch add sanity cleanup logic which looks follow:
     - On sigterm received by parent, it resend signal to it's children
     - Wait for each child to terminates
     - EXTRA_SANITY: Even if parent was killed by other signal, children
       will be terminated with SIGKILL to preven staled children.

    So now one can simply run fsstress like this:
    ./fsstress -p 1000 -n999999999 -d $TEST_DIR &
    PID=$!
    sleep 300
    kill $PID
    wait $PID

The patch has been modified be suitable for LTP and tested.

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@redhat.com>