demand_paging/backing_store/ram.c: misc fixups

First, do align the buffer. The slab code puts pointers in there and it
does not like it if those are not properly aligned.

And return the actual error code from k_mem_slab_alloc() even if errors
shouldn't happen (it did happen to me because of the above ... with
assertions disabled).

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>