f2fs: limit b_size of mapped bh in f2fs_map_bh

			struct extent_info *ei, struct buffer_head *bh_result)

{

	unsigned int blkbits = sb->s_blocksize_bits;

	size_t count;

	size_t max_size = bh_result->b_size;

	size_t mapped_size;

	clear_buffer_new(bh_result);

	map_bh(bh_result, sb, ei->blk + pgofs - ei->fofs);

	count = ei->fofs + ei->len - pgofs;

	if (count < (UINT_MAX >> blkbits))

		bh_result->b_size = (count << blkbits);

	else

		bh_result->b_size = UINT_MAX;

	mapped_size = (ei->fofs + ei->len - pgofs) << blkbits;

	bh_result->b_size = min(max_size, mapped_size);

}

static bool lookup_extent_info(struct inode *inode, pgoff_t pgofs,