struct dentry: make ->d_u anonymous

struct dentry: make ->d_u anonymous

Making ->d_rcu and (then) ->d_child overlapping dates back to
2006; anon unions support had been added to gcc only in 4.6
(2011) and the minimal gcc version hadn't been bumped to that
until 4.19 (2018).

These days there's no reason not to keep that union named.

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