ext4_add_entry(): ->d_name.len is never 0

That bogosity goes back to the initial merge of ext3.  Once upon a time
ext2 used to have a similar check; that got taken out during the switch
to page cache (June 2001).  ext3 got merged into mainline 5 months later,
still using buffer cache for directories; removal of the pointless check
in ext2 should've been done as a separate patch, but it hadn't been,
so that thing got missed...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>