Does IOPortal consider to support to create memory aligned block for direct IO

1. IOPortal calls `pappend_from_file_descriptor` to read data from a opened `fd`, where in my condition `fd` is opened with `O_DIRECT` flag, but the function failed with `invalid argument`. I am sure that the offset and size are chunk-aligned, so I guess this is because the `memory buffer` is not chunk-aligned.
2. I read `iobuf.cpp`, it call `blockmem_allocate` (default is `::malloc`) to alloc memory for `create_block`, which is not memory-aligned for `direct io` operations. This may be the reason for invalid argument.
3. Do we have plan for memory-aligned block allocation or in deed is there a way to achieve this goal. I want to minimize the data copy as fewer as possible.

Read iobuf.cpp and trace pappend_from_file_descriptor through create_block and blockmem_allocate. Check how the memory buffer is allocated for O_DIRECT reads and whether the existing path exposes alignment requirements. Done means documenting or implementing a supported aligned-allocation path that avoids unnecessary copying, with its behavior verified for direct I/O.

Written by the indexing model from the issue text.