afs: Handle a short write to an AFS page

Handle the situation where afs_write_begin() is told to expect that a
full-page write will be made, but this doesn't happen (EFAULT, CTRL-C,
etc.), and so afs_write_end() sees a partial write took place.  Currently,
no attempt is to deal with the discrepency.

Fix this by loading the gap from the server.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/fsclient.c

@@ -393,8 +393,10 @@ static int afs_deliver_fs_fetch_data(struct afs_call *call)
 			if (req->remain > 0) {
 				call->offset = 0;
 				req->index++;
-				if (req->index >= req->nr_pages)
+				if (req->index >= req->nr_pages) {
+					call->unmarshall = 4;
 					goto begin_discard;
+				}
 				goto begin_page;
 			}
 		}