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/internal.h

@@ -130,7 +130,7 @@ struct afs_call_type {
  */
 struct afs_read {
 	loff_t			pos;		/* Where to start reading */
-	loff_t			len;		/* How much to read */
+	loff_t			len;		/* How much we're asking for */
 	loff_t			actual_len;	/* How much we're actually getting */
 	atomic_t		usage;
 	unsigned int		remain;		/* Amount remaining */