mlx4_core: mtts resources units changed to offset

In the previous implementation mtts are managed by:
1. order     - log(mtt segments), 'mtt segment' groups several mtts together.
2. first_seg - segment location relative to mtt table.
In the current implementation:
1. order     - log(mtts) rather than segments
2. offset    - mtt index in mtt table

Note: The actual mtt allocation is made in segments but it is
      transparent to callers.

Rational: The mtt resource holders are not interested on how the allocation
          of mtt is done, but rather on how they will use it.

Signed-off-by: Marcel Apfelbaum <marcela@dev.mellanox.co.il>
Reviewed-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>