RDMA: Remove the udata parameter from alloc_mr callback

Allocating an MR flow can only be initiated by kernel users, and not from
userspace so a udata parameter is redundant.

Link: https://lore.kernel.org/r/20200706120343.10816-4-galpress@amazon.com
Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>