mm/migrate_device.c: fix a misleading and outdated comment

mm/migrate_device.c: fix a misleading and outdated comment

Commit ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED") changed
the way trylock_page() in migrate_vma_collect_pmd() works without updating
the comment.  Reword the comment to be less misleading and a better
reflection of what happens.

Link: https://lkml.kernel.org/r/20220830020138.497063-1-apopple@nvidia.com
Fixes: ab09243aa95a ("mm/migrate.c: remove MIGRATE_PFN_LOCKED")
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reported-by: Peter Xu <peterx@redhat.com>
Acked-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>