ext4: Always set dx_node's fake_dirent explicitly.

When ext4_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.

Signed-off-by: Andreas Schlick <schlick@lavabit.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>