misc: cxl: use mmgrab

Mmgrab was introduced in commit f1f1007644 ("mm: add new mmgrab()
helper") and most of the kernel was updated to use it. Update a
remaining file.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

<smpl>
@@ expression e; @@
- atomic_inc(&e->mm_count);
+ mmgrab(e);
</smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1577634178-22530-2-git-send-email-Julia.Lawall@inria.fr

drivers/misc/cxl/context.c

@@ -352,7 +352,7 @@ void cxl_context_free(struct cxl_context *ctx)
 void cxl_context_mm_count_get(struct cxl_context *ctx)
 {
 	if (ctx->mm)
-		atomic_inc(&ctx->mm->mm_count);
+		mmgrab(ctx->mm);
 }
 
 void cxl_context_mm_count_put(struct cxl_context *ctx)