f2fs: fix wrong block mapping for multi-devices

Assuming the disk layout as below,

disk0: 0            --- 0x00035abfff
disk1: 0x00035ac000 --- 0x00037abfff
disk2: 0x00037ac000 --- 0x00037ebfff

and we want to read data from offset=13568 having len=128 across the block
devices, we can illustrate the block addresses like below.

0 .. 0x00037ac000 ------------------- 0x00037ebfff, 0x00037ec000 -------
          |          ^            ^                                ^
          |   fofs   0            13568                            13568+128
          |       ------------------------------------------------------
          |   LBA    0x37e8aa9    0x37ebfa9                        0x37ec029
          --- map    0x3caa9      0x3ffa9

In this example, we should give the relative map of the target block device
ranging from 0x3caa9 to 0x3ffa9 where the length should be calculated by
0x37ebfff + 1 - 0x37ebfa9.

In the below equation, however, map->m_pblk was supposed to be the original
address instead of the one from the target block address.

 - map->m_len = min(map->m_len, dev->end_blk + 1 - map->m_pblk);

Cc: stable@vger.kernel.org
Fixes: 71f2c82 ("f2fs: multidevice: support direct IO")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: Jaegeuk Kim

fs/f2fs/data.c

@@ -1497,8 +1497,8 @@ static bool f2fs_map_blocks_cached(struct inode *inode,
 		struct f2fs_dev_info *dev = &sbi->devs[bidx];
 
 		map->m_bdev = dev->bdev;
-		map->m_pblk -= dev->start_blk;
 		map->m_len = min(map->m_len, dev->end_blk + 1 - map->m_pblk);
+		map->m_pblk -= dev->start_blk;
 	} else {
 		map->m_bdev = inode->i_sb->s_bdev;
 	}