IB/mlx5: Use fragmented QP's buffer for in-kernel users

The current implementation of create QP requires contiguous memory, such a
requirement is problematic once the memory is fragmented or the system is
low in memory, it causes failures in dma_zalloc_coherent().

This patch takes advantage of the new mlx5_core API which allocates a
fragmented buffer. This makes the QP creation much more resilient to
memory fragmentation. Data-path code was adapted to the fact that WQEs can
cross buffers.

We also use the opportunity to fix some cosmetic legacy coding convention
errors which were in the feature scope.

Signed-off-by: Guy Levi <guyle@mellanox.com>
Reviewed-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>