bfs_add_entry(): get rid of pointless ->d_name.len checks

First of all, any dentry getting here would have passed bfs_lookup(),
so it it passed ENAMETOOLONG check there, there's no need to
repeat it.  And we are not going to get dentries with zero name length -
that check ultimately comes from ext2 and it's as pointless here as it
used to be there.

Acked-by: Tigran Aivazian <aivazian.tigran@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>