Pull up following revision(s) (requested by maya in ticket #1350):
sys/external/bsd/drm2/dist/drm/ttm/ttm_tt.c: revision 1.8
Guarantee no zero-size uao/kmem allocations via ttm.
It may be that all callers guarantee no zero-size ttm objects, but I
can't prove that in five minutes of browsing callers.  Rather than
add a KASSERT, lacking proof, we'll add a warning message so that if
it does happen then it happens noisily, but we'll also prevent the
bad consequences of passing zero into uao_create by rounding up to a
harmless nonzero allocation.

(snj)