[Async] Gracefully degrade io_uring slab registration on RLIMIT_MEMLOCK (#23654)

When IORING_REGISTER_BUFFERS[_UPDATE] fails with ENOMEM (kernel cannot
pin pages due to RLIMIT_MEMLOCK), fall back to copy-based I/O instead of
returning a hard error. The region is still created with full pool
management support, but the send path uses regular sends instead of
SEND_ZC with fixed buffer indices.

This fixes a flake on arm64 CI runners where RLIMIT_MEMLOCK is too low
for the 1MB buffer registration in ZeroCopySendRegisteredLargeTransfer.

ci-extra: all

Co-authored-by: Claude <noreply@anthropic.com>