ext4: fix assertion in ext4_add_complete_io()

It doesn't make sense to require io_end->handle when we are in
nojournal mode. So update the assertion accordingly to avoid false
warnings from ext4_add_complete_io().

Reported-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>