Simplify the condition to enable superpage mappings in vm_fault_soft_fast().

The list of arches list there matches the list of arches where
default VM_NRESERVLEVEL > 0. Before sparc64 removal, that was the
only arch that defined VM_NRESERVLEVEL > 0 to help with cache coloring,
but did not implemented superpages. Now it can be simplified.

Submitted by: alc
Reviewed by: markj