mm/memory-failure.c: remove PageSlab check in hwpoison_filter_dev

mm/memory-failure.c: remove PageSlab check in hwpoison_filter_dev

Since commit 03e5ac2fc3bf ("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.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>