libfabric: Use desc-specific target offset

[tsg-]

What?

This fixes a bug in multi-descriptor transfers where descriptors point to different offsets within the same registered memory region. The bug caused all descriptors to incorrectly use the base address of the registration (remote_md->remote_buf_addr_) instead of each descriptor's specific offset address (remote[desc_idx].addr).

Impact: Block-based transfers (Iteration N would read blocks from iteration 0, etc). Also, Scatter-gather operations, Partial buffer updates.

Why?

Without this fix, RDMA reads always target offset 0. Should extract each descriptor's specific target address instead.

Example test case:

  buffer = allocate_memory(1GB)
  register_memory(buffer)

  # Pass 0: Transfer blocks 0-127
  descriptors = [
      {addr: buffer + 0*block_size, len: block_size},      # Block 0
      {addr: buffer + 1*block_size, len: block_size},      # Block 1
      ...
      {addr: buffer + 127*block_size, len: block_size}     # Block 127
  ]

  # Pass 1: Transfer blocks 128-255
  descriptors = [
      {addr: buffer + 128*block_size, len: block_size},    # Block 128  .... Bug: reads block 0
      {addr: buffer + 129*block_size, len: block_size},    # Block 129  .... Bug: reads block 1
      ...
  ]

How?

After fix: Each descriptor uses remote[desc_idx].addr (specific target offset)

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[github-actions]

👋 Hi tsg-! Thank you for contributing to ai-dynamo/nixl.

Your PR reviewers will review your contribution then trigger the CI to test your changes.

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[tsg-]

@akkart-aws @yexiang-aws we'll push some focused tests for the failing scenarios if it helps with this review. Thank you!

[ovidiusm]

/build

[ovidiusm]

Approving from my side, but it will still need code owner approval from AWS team

[tsg-]

@akkart-aws @yexiang-aws any comments on this change?

[ovidiusm]

/ok to test 005d3ec