IB/mlx5: Fix reqlen validation in mlx5_ib_alloc_ucontext

IB/mlx5: Fix reqlen validation in mlx5_ib_alloc_ucontext

Older libraries that don't have all the new req_v2 fields
should be able to work as well. Today, if the library uses v2, it
will fail to allocate context since the size of reqlen is smaller
than the req_v2 size.

Fix the validation to be with the original req_v2 size and not
the current.

Fixes: 708ef830e7c9 ('IB/mlx5: Expose CQE version to user-space')
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>