generic/561: kill duperemove after sleep_time

generic/561: kill duperemove after sleep_time

generic/561 can take a very long time to run on XFS (45+ minutes)
because it kicks off fsstress and a lot of duperemove processes,
waits 50 seconds, and then waits for the duperemove processes to
finish.  duperemove, however, fights with fsstress for file locks
and can take a very long time to make even a single pass over the
filesystem and exit, which means the test just takes forever to run.

Once we've decided to tear down the duperemove processes let's just send
them SIGINT and then wait for them to exit.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>