MFC:

r246881 | ian | 2013-02-16 13:43:16 -0700 (Sat, 16 Feb 2013) | 4 lines

In _bus_dmamap_addseg(), the return value must be zero for error, or the size
actually added to the segment (possibly smaller than the requested size if
boundary crossings had to be avoided).

This fixes NFS root on Atmel platforms, and likely others.