netfs: Rearrange netfs_io_subrequest to put request pointer first

Rearrange the netfs_io_subrequest struct to put the netfs_io_request
pointer (rreq) first. This then allows netfs_io_subrequest to be put in a
union with a pointer to a wrapper around netfs_io_request. This will be
useful in the future for cifs and maybe ceph.

Signed-off-by: David Howells <[e-mail]>
cc: Steve French <[e-mail]>
cc: Shyam Prasad N <[e-mail]>
cc: Rohith Surabattula <[e-mail]>
cc: Jeff Layton <[e-mail]>
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]

include/linux/netfs.h

···
  * the pages it points to can be relied on to exist for the duration.
  */
 struct netfs_io_subrequest {
-	struct work_struct	work;
 	struct netfs_io_request *rreq;		/* Supervising I/O request */
+	struct work_struct	work;
 	struct list_head	rreq_link;	/* Link in rreq->subrequests */
 	struct iov_iter		io_iter;	/* Iterator for this subrequest */
 	loff_t			start;		/* Where to start the I/O */