IB/ipath: Implement new verbs DMA mapping functions

This patch implements the interposing DMA mapping functions to allow
support for IOMMUs and remove the dependence on phys_to_virt() and
bus_to_virt().

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>