RDMA/mlx5: Silence the overflow warning while building offset mask

Coverity reports "Potentially overflowing expression ..." warning, which
is correct thing to complain from the compiler point of view, but this is
not possible in the current code. Still, this is a small error as there
are some future situations that might need to use a 32 bit offset. Use ULL
so the calculation works up to 63.

Fixes: b045db62 ("RDMA/mlx5: Use ib_umem_find_best_pgoff() for SRQ")
Link: https://lore.kernel.org/r/20201125061704.6580-1-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>