mm/alpha: unify mem_init() for both UMA and NUMA architectures

Now mem_init() for both Alpha UMA and Alpha NUMA are the same, so unify it
to reduce duplicated code.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>