vdpa/mlx5: Fix possible failure in umem size calculation

vdpa/mlx5: Fix possible failure in umem size calculation

[ Upstream commit 71ab6a7cfbae27f86a3901daab10bfe13b3a1e3a ]

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: 1a86b377aa21 ("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>