nvme-rdma: introduce nvme_rdma_sgl structure

Remove first_sgl pointer from struct nvme_rdma_request and use pointer
arithmetic instead. The inline scatterlist, if exists, will be located
right after the nvme_rdma_request. This patch is needed as a preparation
for adding PI support.

Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>