uma: unify layout paths and improve efficiency

Unify the keg layout selection paths (keg_small_init, keg_large_init,
keg_cachespread_init), and slightly improve memory efficiecy by:

• using the padding of the final item to store the slab header,
• not going OFFPAGE if we have a choice unless it improves efficiency.

Reviewed by: jeff, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23048