mm: set S_SWAPFILE on blockdev swap devices

mainline inclusion
from mainline-5.4-rc1
commit 1638045c
category: bugfix
bugzilla: 50612
CVE: NA
---------------------------

Set S_SWAPFILE on block device inodes so that they have the same
protections as a swap flie.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: zhangyi (F) <yi.zhang@huawei.com>
Reviewed-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>