mm/migrate_device.c: fix a misleading and outdated comment

		get_page(page);

		/*

		 * Optimize for the common case where page is only mapped once

		 * in one process. If we can lock the page, then we can safely

		 * set up a special migration page table entry now.

		 * We rely on trylock_page() to avoid deadlock between

		 * concurrent migrations where each is waiting on the others

		 * page lock. If we can't immediately lock the page we fail this

		 * migration as it is only best effort anyway.

		 *

		 * If we can lock the page it's safe to set up a migration entry

		 * now. In the common case where the page is mapped once in a

		 * single process setting up the migration entry now is an

		 * optimisation to avoid walking the rmap later with

		 * try_to_migrate().

		 */

		if (trylock_page(page)) {

			bool anon_exclusive;