RDMA/mlx5: Refactor mlx5_post_send() to improve readability

Add small helpers in order to avoid code duplication and improve code
readability. Decrease the amount of code in the gigantic post_send
function and divide it to readable methods that will help in code
maintenance in the future.

Link: https://lore.kernel.org/r/20200506065513.4668-3-leon@kernel.org
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>