generic/019: kill background processes on interrupt

generic/019: kill background processes on interrupt

If you ctrl-c generic/019, it leaves fsstress processes running.
Kill them in the cleanup function so that they don't have to be
manually killed after interrupting the test.

While touching the _cleanup() function, make it do everything that
the generic _cleanup function it overrides does and fix the
indenting.

[Eryu: unset fs_pid and fio_pid after wait]

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>