nvme: create helper function to obtain command effects

Separate the code to obtain command effects from the code
to start a passthru request and move the nvme_passthru_start() and
nvme_passthru_end() functions up above nvme_submit_user_cmd() in order
that they may be used in a new helper a subsequent patch.

The new helper function will be necessary for nvmet passthru
code to determine if we need to change out of interrupt context
to handle the effects. It is exported in the NVME_TARGET_PASSTHRU
namespace.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>