jffs2: Fix return value from jffs2_do_readpage_nolock()

This fixes "kernel BUG at fs/jffs2/file.c:251!".
This pseudocode hopefully illustrates the scenario that triggers it:

jffs2_write_begin {
     jffs2_do_readpage_nolock {
         jffs2_read_inode_range {
             jffs2_read_dnode {
                 Data CRC 33c102e9 != calculated CRC 0ef77e7b for node at 005d42e4
                 return -EIO;
             }
         }
         ClearPageUptodate(pg);
         return 0;
     }
}

jffs2_write_end {
     BUG_ON(!PageUptodate(pg));
}

Signed-off-by: Anders Grafström <grfstrm@users.sourceforge.net>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>