afs: switch dynroot lookups to d_splice_alias()

->lookup() methods can (and should) use d_splice_alias() instead of
d_add().  Even if they are not going to be hit by open_by_handle(),
code does get copied around...

Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>