mshv: Add __user attribute to argument passed to access_ok()

access_ok() expects its first argument to have the __user attribute
since it is checking access to user space. Current code passes an
argument that lacks that attribute, resulting in 'sparse' flagging
the incorrect usage. However, the compiler doesn't generate code
based on the attribute, so there's no actual bug.

In the interest of general correctness and to avoid noise from sparse,
add the __user attribute. No functional change.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512141339.791TCKnB-lkp@intel.com/
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>