mm/swap.c: fix incomplete comment in lru_cache_add_inactive_or_unevictable()

mm/swap.c: fix incomplete comment in lru_cache_add_inactive_or_unevictable()

Since commit 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping
pagevecs"), unevictable pages do not goes directly back onto zone's
unevictable list.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Shakeel Butt <shakeelb@google.com>
Link: https://lkml.kernel.org/r/20200927122209.59328-1-linmiaohe@huawei.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>