RDMA/mlx5: Split the WR setup out of mlx5_ib_update_xlt()

The memory allocation is quite complicated, and makes this function hard
to understand. Refactor things so that a function call sets up the WR, SG,
DMA mapping and buffer, further splitting that into buffer and DMA/wr.

This also slightly changes the buffer allocation logic to try an order 0
page allocation (with OOM warnings on) before going to the emergency page.

Link: https://lore.kernel.org/r/20201026132314.1336717-4-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>