Method alloc_inner

The inner function that do the real segment allocation. Support two extended parameters: 1. `offset_within_first_block`: the offset (in bytes) within the first block. 2. `len`: the exact length (in bytes) of the wanted segment. (May less than `nblocks * BLOCK_SIZE`) # Panics If the `offset_within_first_block` or `len` is not sector aligned, this method will panic.

(
        nblocks: usize,
        offset_within_first_block: usize,
        len: usize,
        direction: BioDirection,
    )