devres: handle zero size in devm_kmalloc()

Make devm_kmalloc() behave similarly to non-managed kmalloc(): return
ZERO_SIZE_PTR when requested size is 0. Update devm_kfree() to handle
this case.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200629065008.27620-5-brgl@bgdev.pl


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>