misc: cxl: use mmgrab

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 <Julia.Lawall@inria.fr>
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Link: https://lore.kernel.org/r/1577634178-22530-2-git-send-email-Julia.Lawall@inria.fr
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>