generic/038: Avoid EBUSY errors on umount

generic/038: Avoid EBUSY errors on umount

Although the test waits for running subshells after sending SIGTERM
signal to them, it does not wait for subprocesses of those subshells
properly. Thus we can hit EBUSY errors when umount is called. Make
subshells wait for executed subprocesses when receiving SIGTERM to avoid
the problem.

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>