[iobuf] Ensure I/O buffer data sits within unshared cachelines

[iobuf] Ensure I/O buffer data sits within unshared cachelines

On platforms where DMA devices are not in the same coherency domain as
the CPU cache, we must ensure that DMA I/O buffers do not share
cachelines with other data.

Align the start and end of I/O buffers to IOB_ZLEN, which is larger
than any cacheline size we expect to encounter.

Signed-off-by: Michael Brown <mcb30@ipxe.org>