IB/mlx4: Implement FMRs

Implement FMRs for mlx4.  This is an adaptation of code from mthca.

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