x86-64, NUMA: Clean up initmem_init()

This patch cleans initmem_init() so that it is more readable and doesn't
use an unnecessary array of function pointers to convolute the flow of
the code.  It also makes it obvious that dummy_numa_init() will always
succeed (and documents that requirement) so that the existing BUG() is
never actually reached.

No functional change.

-tj: Updated comment for dummy_numa_init() slightly.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>