uma: convert mbuf_jumbo_alloc to UMA_ZONE_CONTIG & tag others

Remove mbuf_jumbo_alloc and let large mbuf zones use the new uma default
contig allocator (a copy of mbuf_jumbo_alloc). Tag other zones which
require contiguous objects, even if they don't use the new default
contig allocator, so that uma knows about their constraints.

Reviewed by: jeff, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23238