rust/nfs: fix read reply handling

rust/nfs: fix read reply handling

READ replies with large data chunks are processed partially to avoid
queuing too much data. When the final chunk was received however, the
start of the chunk would already tag the transaction as 'done'. The
more aggressive tx freeing that was recently merged would cause this
tx to be freed before the rest of the in-progress chunk was done.

This patch delays the tagging of the tx until the final data has been
received.