nvme-fc: Ensure private pointers are NULL if no data

Ensure that when allocations are done, and the lldd options indicate
no private data is needed, that private pointers will be set to NULL
(catches driver error that forgot to set private data size).

Slightly reorg the allocations so that private data follows allocations
for LS request/response buffers. Ensures better alignments for the buffers
as well as the private pointer.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>