mm/munlock: delete smp_mb() from __pagevec_lru_add_fn()

	VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);

	folio_set_lru(folio);

	/*

	 * A folio becomes evictable in two ways:

	 * 1) Within LRU lock [munlock_vma_page() and __munlock_pagevec()].

	 * 2) Before acquiring LRU lock to put the folio on the correct LRU

	 *    and then

	 *   a) do PageLRU check with lock [check_move_unevictable_pages]

	 *   b) do PageLRU check before lock [clear_page_mlock]

	 *

	 * (1) & (2a) are ok as LRU lock will serialize them. For (2b), we need

	 * following strict ordering:

	 *

	 * #0: __pagevec_lru_add_fn		#1: clear_page_mlock

	 *

	 * folio_set_lru()			folio_test_clear_mlocked()

	 * smp_mb() // explicit ordering	// above provides strict

	 *					// ordering

	 * folio_test_mlocked()			folio_test_lru()

	 * Is an smp_mb__after_atomic() still required here, before

	 * folio_evictable() tests PageMlocked, to rule out the possibility

	 * of stranding an evictable folio on an unevictable LRU?  I think

	 * not, because munlock_page() only clears PageMlocked while the LRU

	 * lock is held.

	 *

	 *

	 * if '#1' does not observe setting of PG_lru by '#0' and

	 * fails isolation, the explicit barrier will make sure that

	 * folio_evictable check will put the folio on the correct

	 * LRU. Without smp_mb(), folio_set_lru() can be reordered

	 * after folio_test_mlocked() check and can make '#1' fail the

	 * isolation of the folio whose mlocked bit is cleared (#0 is

	 * also looking at the same folio) and the evictable folio will

	 * be stranded on an unevictable LRU.

	 * (That is not true of __page_cache_release(), and not necessarily

	 * true of release_pages(): but those only clear PageMlocked after

	 * put_page_testzero() has excluded any other users of the page.)

	 */

	folio_set_lru(folio);

	smp_mb__after_atomic();

	if (folio_evictable(folio)) {

		if (was_unevictable)

			__count_vm_events(UNEVICTABLE_PGRESCUED, nr_pages);