RDMA/core: Remove ib_alloc_mr_user function

Allocating an MR flow can only be initiated by kernel users, and not from
userspace. As a result, the udata parameter is always being passed as
NULL. Rename ib_alloc_mr_user function to ib_alloc_mr and remove the udata
parameter.

Link: https://lore.kernel.org/r/20200706120343.10816-3-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>