mnt_want_write() and mnt_want_write_file() will check sb->s_flags with
MS_RDONLY, and we don't need to do it ourselves.
Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
                                                    u64 *transid,
                                                    bool readonly)
 {
-       struct btrfs_root *root = BTRFS_I(fdentry(file)->d_inode)->root;
        struct file *src_file;
        int namelen;
        int ret = 0;
 
-       if (root->fs_info->sb->s_flags & MS_RDONLY)
-               return -EROFS;
-
        ret = mnt_want_write_file(file);
        if (ret)
                goto out;
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;
 
-       if (fs_info->sb->s_flags & MS_RDONLY)
-               return -EROFS;
-
        ret = mnt_want_write(file->f_path.mnt);
        if (ret)
                return ret;