UPSTREAM PR #17116: rpc : fix alloc size logic

[DajanaV]

Mirrored from ggml-org/llama.cpp#17116

fix #16657
ref ggml-org/llama.cpp#16276 (review)

This fixes the RPC inference when Metal backend is involved.

Testing:

# server
make -j && ./bin/rpc-server

# cli
make -j && ./bin/llama-cli -m ../models/gemma-3-4b-it/ggml-model-f16.gguf --rpc localhost:50052 -ngl 99 --no-mmap -no-cnv -p "Hello" --top-k 1 -n 32 -fa on

TODO:

• Check performance imapct
• Cache the responses to avoid extra RPC calls?