nvme: move command clear into the various setup helpers

We don't have to worry about doing extra memsets by moving it outside
the protection of RQF_DONTPREP, as nvme doesn't do partial completions.

This is in preparation for making the read/write fast path not do a full
memset of the command.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>