libceph: distinguish page and bio requests

An osd request uses either pages or a bio list for its data.  Use a
union to record information about the two, and add a data type
tag to select between them.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

fs/ceph/file.c

@@ -571,6 +571,7 @@ more:
 			req->r_data.own_pages = 1;
 		}
 	}
+	req->r_data.type = CEPH_OSD_DATA_TYPE_PAGES;
 	req->r_data.pages = pages;
 	req->r_data.num_pages = num_pages;
 	req->r_data.alignment = page_align;