Fix limit testing after 1762f674ccb571e6 ktrace commit.

The previous:

if ((uoff_t)uio->uio_offset + uio->uio_resid > lim)
signal(....);

was replaced with:

if ((uoff_t)uio->uio_offset + uio->uio_resid < lim)
return;
signal(....);

Making (uoff_t)uio->uio_offset + uio->uio_resid == lim trip over the
limit, when it did not previously.

Unbreaks running 13.0 buildworld.