nilfs2: emit error message when I/O error is detected

When nilfs returned -EIO as an error code, it's not always clear if it
came from the underlying block device or not.  This will mend the issue
by having low level I/O routines of nilfs output an error message when
they detected an I/O error.

Link: http://lkml.kernel.org/r/1464875891-5443-7-git-send-email-konishi.ryusuke@lab.ntt.co.jp
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nilfs2/mdt.c

@@ -207,8 +207,12 @@ static int nilfs_mdt_read_block(struct inode *inode, unsigned long block,
 
  out_no_wait:
 	err = -EIO;
-	if (!buffer_uptodate(first_bh))
+	if (!buffer_uptodate(first_bh)) {
+		nilfs_msg(inode->i_sb, KERN_ERR,
+			  "I/O error reading meta-data file (ino=%lu, block-offset=%lu)",
+			  inode->i_ino, block);
 		goto failed_bh;
+	}
  out:
 	*out_bh = first_bh;
 	return 0;