f2fs: invalidate block device page cache on umount

Neither F2FS nor VFS invalidates the block device page cache, which
results in reading stale metadata. An example scenario is shown below:

Terminal A Terminal B
mount /dev/vdb /mnt/f2fs
touch mx // ino = 4
sync
dump.f2fs -i 4 /dev/vdb// block on "[Y/N]"
touch mx2 // ino = 5
sync
umount /mnt/f2fs
dump.f2fs -i 5 /dev/vdb // block addr is 0

After umount, the block device page cache is not purged, causing
`dump.f2fs -i 5 /dev/vdb` to read stale metadata and see inode 5 with
block address 0.

Btrfs has encountered a similar issue before, the solution there was to
call sync_blockdev() and invalidate_bdev() when the device is closed:

mail-archive.com/linux-btrfs@vger.kernel.org/msg54188.html

For the root user, the f2fs kernel calls sync_blockdev() on umount to
flush all cached data to disk, and f2fs-tools can release the page cache
by issuing ioctl(fd, BLKFLSBUF) when accessing the device. However,
non-root users are not permitted to drop the page cache, and may still
observe stale data.

This patch calls sync_blockdev() and invalidate_bdev() during umount to
invalidate the block device page cache, thereby preventing stale
metadata from being read.

Note that this may result in an extra sync_blockdev() call on the first
device, in both f2fs_put_super() and kill_block_super(). The second call
do nothing, as there are no dirty pages left to flush. It ensures that
non-root users do not observe stale data.

Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>