IB/mlx5: Use u64 for UMR length

The fast_registration length is used to convey length for memory
registrations through UMR which can be of any size up to 2^64.

Change the length type to be u64.

Fixes: 968e78dd ('IB/mlx5: Enhance UMR support to allow partial page table update')
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>