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

The current implementation of create SRQ 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 the advantage of the new mlx5_core API which allocates a
fragmented buffer, and makes the SRQ creation much more resilient to
memory fragmentation. Data-path code was adapted to the fact that WQEs can
cross buffers.

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