cxl: don't manipulate the mm.mm_users field directly

It is better to rely on the API provided by the MM layer instead of
directly manipulating the mm_users field.

Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210310174405.51044-1-ldufour@linux.ibm.com