use superpage reservations.  So, for the first time, kernel virtual memory
that is allocated by contigmalloc(), kmem_alloc_attr(), and
kmem_alloc_contig() can be promoted to superpages.  In fact, even a series
of small contigmalloc() allocations may collectively result in a promoted
superpage.

Eliminate some duplication of code in vm_reserv_alloc_page().

Change the type of vm_reserv_reclaim_contig()'s first parameter in order
that it be consistent with other vm_*_contig() functions.

Tested by:	marius (sparc64)