nvme: Allow command opcodes with lower bit clear as reads

[imp]

Summary

Linux does this, and nvme-cli has some vendor specific opcodes that need
this.

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[chuck]

sys/dev/nvme/nvme_ctrlr.c

1371

So now when the OP code's data transfer field says there is no data and this is a "user command" (all the instances of nvme_ctrlr_linux_passthru_cmd() appear to set this to true), the driver is going to allocate npc->data_len worth of memory? Any concerns about the value of data_len potentially being set for a command that wouldn't normally transfer data?