Utilize the new function kmem_alloc_contig() to implement the UMA back-end

Utilize the new function kmem_alloc_contig() to implement the UMA back-end
allocator for the jumbo frames zones.  This change has two benefits: (1) a
custom back-end deallocator is no longer required.  UMA's standard
deallocator suffices. (2) It eliminates a potentially confusing artifact
of using contigmalloc(): The malloc(9) statistics contain bogus information
about the usage of jumbo frames.  Specifically, the malloc(9) statistics
report all jumbo frames in use whereas the UMA zone statistics report the
"truth" about the number in use vs. the number free.