RDMA/mlx4: Use ib_umem_num_dma_blocks()

RDMA/mlx4: Use ib_umem_num_dma_blocks()

For the calls linked to mlx4_ib_umem_calc_optimal_mtt_size() use
ib_umem_num_dma_blocks() inside the function, it is just some weird static
default.

All other places are just using it with PAGE_SIZE, switch to
ib_umem_num_dma_blocks().

As this is the last call site, remove ib_umem_num_count().

Link: https://lore.kernel.org/r/15-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.com
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>