generic/299: truncate can fail with ENOSPC

generic/299: truncate can fail with ENOSPC

So this test does lots of fallocate/truncate noise while doing aio
overwrites to try and exercise a deadlock found in ext4.  Because it
runs so hard with ENOSPC it can sometimes cause truncate to fail on
btrfs.  This is ok and doesn't affect the validity of the test, we
just need to catch the output so it doesn't cause the test to fail.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>