Register 'open' operation in `simplefs_file_ops`

[jserv]

In the registration of simplefs_file_ops callbacks, there is no open operation, which may lead to creating unexpected space. We can specify our own open operation by truncating, and here is a possible implementation:

/*
 * Called when a file is opened in the simplefs.
 * It checks the flags associated with the file opening mode (O_WRONLY, O_RDWR, O_TRUNC)
 * and performs truncation if the file is being opened for write or read/write and
 * the O_TRUNC flag is set.
 *
 * Truncation is achieved by reading the file's index block from disk, iterating
 * over the data block pointers, releasing the associated data blocks, and updating
 * the inode metadata (size and block count).
 * 
static int simplefs_open(struct inode *inode, struct file *file)                                                                                              
{
    bool wronly = (bool) (file->f_flags & O_WRONLY);
    bool rdwr = (bool) (file->f_flags & O_RDWR);
    bool trunc = (bool) (file->f_flags & O_TRUNC);

    if ((wronly || rdwr) && trunc && inode->i_size) {
        struct super_block *sb = inode->i_sb;
        struct simplefs_sb_info *sbi = SIMPLEFS_SB(sb);
        struct simplefs_inode_info *ci = SIMPLEFS_INODE(inode);
        struct simplefs_file_index_block *index;
        struct buffer_head *bh_index;
        sector_t iblock;

        /* Fetch the file's index block from disk */
        bh_index = sb_bread(sb, ci->index_block);
        if (!bh_index)
            return -EIO;
        index = (struct simplefs_file_index_block *) bh_index->b_data;

        for (iblock = 0; index->blocks[iblock]; iblock++) {
            put_block(sbi, index->blocks[iblock]);
            index->blocks[iblock] = 0;
        }
        inode->i_size = 0;
        inode->i_blocks = 1;

        brelse(bh_index);
    }

    return 0;
}

const struct file_operations simplefs_file_ops = {
    .owner = THIS_MODULE,
    .open = simplefs_open,
    ...
};