to extract the physical address from the virtual.
On the ARM, also use the "read-only at MMU" indication to avoid a
redundant cache clean operation.
Other platforms should use these two as examples of how to use these
new pool/mbuf features to improve network performance. Note this requires
a platform to provide a working POOL_VTOPHYS().
Part 3 in a series of simple patches contributed by Wasabi Systems
to improve network performance.