f2fs-tools: invalidate block device page cache before reading metadata

After the device is mounted, f2fs updates on-disk metadata without
updating the block device page cache. As a result, f2fs-tools may read
stale metadata from the page cache.

For example:
mount /dev/vdb /mnt/f2fs
touch mx // ino = 4
sync
dump.f2fs -i 4 /dev/vdb
touch mx2 // ino = 5
sync
dump.f2fs -i 5 /dev/vdb // block addr is 0

Since `dump.f2fs -i 4 /dev/vdb` has already populated the metadata area
into the block device page cache, the cached pages remain resident in
memory. After creating mx2, the page cache is not updated accordingly.
Therefore, `dump.f2fs -i 5 /dev/vdb` reads stale metadata, and the
physical block address of inode 5 is incorrectly reported as 0.

This patch issues a BLKFLSBUF ioctl to the block device before reading
metadata. The kernel then invalidates the block device page cache,
ensuring that subsequent reads fetch uptodate metadata from disk.

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

Author: Yongpeng Yang

lib/libf2fs.c

@@ -1016,6 +1016,7 @@ int get_device_info(int i)
 	}
 
 	dev->fd = fd;
+	ioctl(fd, BLKFLSBUF);
 
 	if (c.sparse_mode && i == 0) {
 		if (f2fs_init_sparse_file()) {