IB/mthca: Merge MR and FMR space on 64-bit systems

For Tavor, we currently reserve separate MPT and MTT space for FMRs to
avoid abusing the vmalloc space on 32 bit kernels. No such problem
exists on 64 bit kernels so let's not do it there.

This way we have a shared pool for MR and FMR resources, used on
demand.  This will also make it possible to write MTTs for regular
regions directly from driver.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>