dax: Ensure errno is returned from dax_direct_access

If the caller specifies a negative nr_pages that is an invalid
parameter.

Return -EINVAL to ensure callers get an errno if they want to check it.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210525172428.3634316-4-ira.weiny@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>