target: Shrink struct se_cmd by rearranging fields

On x86-64, these changes reduce the struct size from 528 bytes to 496
bytes.  We save a cacheline and get under 512 bytes for better packing.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Acked-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>