If not compiled with -D_KERNEL, include different includes and
do so macro magic so that this will fit sanely into test harnesses.
When used in user-land, this should be compiled with -D_EXTENT_TESTING.
Bug fixes:
(extent_insert_and_optimize) You can't do things like:
LIST_REMOVE(elem->...le_next, ...);
free(elem->...le_next, ...);
They just don't work (and will corrupt your list and/or malloc free list).
(extent_alloc_region_descriptor) Unless you wait, malloc can fail.
Don't accidentally deref a potentially-NULL pointer.
e77b6879f7