[Virtio-fs] [virtiofsd] MR merged: process_queue_pool: Only acquire the VringMutex lock once

[virtiofs-bot at sinrega.org]

Previously, the worker task in `process_queue_pool()` called
up to 3 functions on `worker_vring` where each function is a
wrapper that first locks the mutex.  This is unneccessary
congestion.  We fix this by locking the mutex once.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/87