Since commit 03e5ac2f ("mm: fix crash when using XFS on loopback"),
page_mapping() can handle the Slab pages.  So remove this unnecessary
PageSlab check and obsolete comment.

Link: https://lkml.kernel.org/r/20220218090118.1105-6-linmiaohe@huawei.comSigned-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Acked-by: NNaoya Horiguchi <naoya.horiguchi@nec.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>