RDMA/mlx5: Merge similar flows of allocating MR from the cache

RDMA/mlx5: Merge similar flows of allocating MR from the cache

When allocating a MR from the cache, the driver calls to get_cache_mr(),
and in case of failure, retries with create_cache_mr(). This is the flow
of mlx5_mr_cache_alloc(), so use it instead.

Link: https://lore.kernel.org/r/53c85fcd4de6ec9de0b8e6cbb1bf5d5fe19900c3.1644947594.git.leonro@nvidia.com
Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>