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.
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Link: https://lore.kernel.org/r/20210310174405.51044-1-ldufour@linux.ibm.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/cxl/fault.c

@@ -200,7 +200,7 @@ static struct mm_struct *get_mem_context(struct cxl_context *ctx)
 	if (ctx->mm == NULL)
 		return NULL;
 
-	if (!atomic_inc_not_zero(&ctx->mm->mm_users))
+	if (!mmget_not_zero(ctx->mm))
 		return NULL;
 
 	return ctx->mm;