Dan Roseberg has reported a problem with the MOVE_EXT ioctl.  If the
donor file is an append-only file, we should not allow the operation
to proceed, lest we end up overwriting the contents of an append-only
file.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Dan Rosenberg <dan.j.rosenberg@gmail.com>