Pull up following revision(s) (requested by christos in ticket #1011):
sys/arch/x86/x86/bus_dma.c: revision 1.72
sys/arch/x86/x86/bus_dma.c: revision 1.73
sys/arch/x86/x86/bus_dma.c: revision 1.74
- If we succeeded allocating a buffer that did not need bouncing before, but
  the buffer in the previous mapping did, clear the bounce bit. Fixes the
  ld_virtio.c bug with machines 8GB and dd if=/dev/zero of=crash bs=1g count=4.
- Allocate with M_ZERO instead of doing memset
- The panic string can take a format, use it.
- When checking for the bounce buffer boundary check addr + len < limit, not
  addr < limit.
make sure we have a cookie before we try to clear it.
fix operator precedence.

(riz)