[sgen] Allow TLABs larger than MAX_SMALL_OBJ_SIZE.

[sgen] Allow TLABs larger than MAX_SMALL_OBJ_SIZE.

`sgen_ensure_free_space()` assumed that if `size` was larger than
`SGEN_MAX_SMALL_OBJ_SIZE`, it should find free space on the major
heap, so it wouldn't consider triggering a nursery allocation.  Making
TLABs larger than that would thus never trigger nursery collections
anymore.

Now we pass in the generation that says where we want space.