samples: configfs: drop unnecessary ternary operators

Checking pointers for NULL value before passing them to container_of()
is pointless because even if we return NULL from the ternary operator,
none of the users checks the returned value but they instead dereference
it unconditionally. AFAICT this cannot really happen either. Simplify
the code by removing the ternary operators from to_childless() et al.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>