vdpa/mlx5: Fix possible failure in umem size calculation

[ Upstream commit 71ab6a7c ]

umem size is a 32 bit unsigned value so assigning it to an int could
cause false failures. Set the calculated value inside the function and
modify function name to reflect the fact it updates the size.

This bug was found during code review but never had real impact to this
date.

Fixes: 1a86b377

 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20210530090349.8360-1-elic@nvidia.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>