generic/077: fall back to /usr if /lib/modules doesn't exist

generic/077: fall back to /usr if /lib/modules doesn't exist

We need a source of "filler" that can fill up a 50M file system.
Unfortunately, on some systems /lib/modules might not exist at all.
In that case, use /usr instead.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>