generic/270: wait for fsstress processes to be killed

generic/270: wait for fsstress processes to be killed

Currently wait will do nothing, because $pid is empty due to the fact
that the command was ran on the background in a separate shell so we
never got the $! set.

This is causing unexpected test failures especially under low memory
due to the fact that the fsstress is still running when we are
checking and comparing quota usage.

Fix it by using -w argument for killall that will wait for all processes
to actually die before it exits.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>