RDMA/mlx5: Re-organize the DM code

1. Inline the checks from check_dm_type_support() into their
   respective allocation functions.
2. Fix use after free when driver fails to copy the MEMIC address to the
   user by moving the allocation code into their respective functions,
   hence we avoid the explicit call to free the DM in the error flow.
3. Split mlx5_ib_dm struct to memic and icm proper typesafety
   throughout.

Fixes: dc2316eb

 ("IB/mlx5: Fix device memory flows")
Link: https://lore.kernel.org/r/20210411122924.60230-5-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>