bad_inode: add missing i_op initializers

New inode operations were forgotten to be added to bad_inode.  Most of the
time the op is checked for NULL before being called but marking the inode
bad and the check can race (very unlikely).

However in case of ->get_link() only DCACHE_SYMLINK_TYPE is checked before
calling the op, so there's no race and will definitely oops when trying to
follow links on such a beast.

Also remove comments about extinct ops.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>