RDMA/mlx5: Unfold modify RMP function

There is no need to perform modify_rmp in two separate function,
while one of them uses stack as a placeholder for data while other
allocates it dynamically. Combine those two functions to one call
instead of two.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>