vmsplice: add vmsplice-to-user support

A bit of a cheat, it actually just copies the data to userspace. But
this makes the interface nice and symmetric and enables people to build
on splice, with room for future improvement in performance.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

include/linux/pipe_fs_i.h

@@ -88,7 +88,13 @@ int generic_pipe_buf_steal(struct pipe_inode_info *, struct pipe_buffer *);
 struct splice_desc {
 	unsigned int len, total_len;	/* current and remaining length */
 	unsigned int flags;		/* splice flags */
-	struct file *file;		/* file to read/write */
+	/*
+	 * actor() private data
+	 */
+	union {
+		void __user *userptr;	/* memory to write to */
+		struct file *file;	/* file to read/write */
+	} u;
 	loff_t pos;			/* file position */
 };