kobj: Use M_WAITOK in kobj_init

[jhb]

Summary

Blocking allocation is safe in all of the current callers of kobj_init
(most of them do a M_WAIT malloc of the structure passed as the first
argument to kobj_init just before calling it). kobj_init doesn't
return an error code but instead panics if the nested malloc in
kobj_class_compile1 fails, so using M_WAITOK here is more robust.