target/configfs: allocate only 6 slots for dev_cg->default_groups

Only slots 0-5 are used so 6 slots should be enough. I don't see anyone
writting anything else than NULL into ->default_groups[5] so a "late"
initialisation should not happen here.

(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>