Side metadata issues

[qinsoon]

This issue tracks problems of our side metadata implementation.

• Side metadata bound #462
• For 64 bits, global side metadata base address is 0x600_0000_0000. For 64 bits, the range for the first space is [0x200_0000_0000, 0x400_0000_0000), for the second space [0x400_0000_0000, 0x600_0000_0000), and for the third space [0x600_0000_0000, 0x800_0000_0000). This means our global side metadata clashes with the third space we have.
• When we unmap side metadata (at the end of tests), we do not tell the mmapper about the unmapping. The mmapper would still think we have mapped those chunks.
• When calculating the side metadata address and size to map (e.g. try_mmap_contiguous_metadata_space()), we are not considering overflow. If we try reserve side metadata for the whole address range (e.g. 32 bits), we will have an arithmetic overflow.
• code duplication between side metadata and header metadata, especially the code that deals with bits and masks for metadata of less than 1 byte.