nvme: avoid memset for passthrough requests

nvme_clear_nvme_request() clears the nvme_command, which is unncessary
for passthrough requests as nvme_command is overwritten immediately.
Move clearing part from this helper to the caller, so that double memset
for passthrough requests is avoided.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>