ext3: Always set dx_node's fake_dirent explicitly.

(crossport of 1f7bebb9
by Andreas Schlick <schlick@lavabit.com>)

When ext3_dx_add_entry() has to split an index node, it has to ensure that
name_len of dx_node's fake_dirent is also zero, because otherwise e2fsck
won't recognise it as an intermediate htree node and consider the htree to
be corrupted.

CC: stable@kernel.org
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>