Fix for buildworld hang on XLR

[C. Jayachandran]

Hi Randall,

Looks like the buildworld hang was caused by a bug in the GMAC driver.
In some cases an additional zero length descriptor was getting added
to the descriptor list which makes the GMAC go into underrun
condition.  Can you see if this fix works for you?

The patch is at http://sites.google.com/site/cjayachandran/files/rge.c.diff

I still get a crash in pmap_enter during buildworld after quite a long
time. It seems that de-referencing the pte in some cases crashes. I'm
looking at it.

Regards,
JC.