mlx4_core: Scale size of MTT table with system RAM

The current driver defaults to 1M MTT segments, where each segment holds
8 MTT entries.  This limits the total memory registered to 8M * PAGE_SIZE
which is 32GB with 4K pages.  Since systems that have much more memory
are pretty common now (at least among systems with InfiniBand hardware),
this limit ends up getting hit in practice quite a bit.

Handle this by having the driver allocate at least enough MTT entries to
cover 2 * totalram pages.

Signed-off-by: Roland Dreier <roland@purestorage.com>