misc: cxl: use mmgrab

Mmgrab was introduced in commit f1f1007644ff ("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 <[e-mail]>
Acked-by: Andrew Donnellan <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/cxl/context.c

···
 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)