target/configfs: use kmalloc() instead of kzalloc() for default groups

All elements are assigned even the NULL member at the end so there is no
reason to allocate zeroed memory.

(nab: Fix up minor apply breakage in for-next)

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>