nvmet: reorder fields in 'struct nvmefc_fcp_req'

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct nvmefc_fcp_req' from
112 to 104 bytes.

This structure is embedded in some other structures (nvme_fc_fcp_op
which itself is embedded in nvme_fcp_op_w_sgl), so it helps reducing the
size of these structures too.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>